Energy-domain synchrotron radiation Mössbauer source for physics under extreme conditions

! ! Energy'domain-synchrotron-radiationMössbauer-source-for-physics-underextreme-conditions- ! ! DISSERTATION! ! ! zur!Erlangung!des!akademischen!Grades!eines! Doktors!der!Naturwissenschaften!(Dr.!rer.!nat.)! an!der!Bayreuther!Graduiertenschule!für!Mathematik!und! Naturwissenschaften!(BayNAT)!der!Universität!Bayreuth! ! ! vorgelegt!von! ! ! Vasily-Potapkin! aus!Ekaterinburg!(Russland)! ! ! ! ! ! Bayreuth,!2012! ! ! Die vorliegende Arbeit wurde in der Zeit von März 2009 bis Juli 2012 am Bayerisches Geoinstitut der Universität Bayreuth unter der Leitung von Herrn Prof. Dr. Leonid Dubrovinsky angefertigt. Vollständiger Abdruck der von der Fakultät für Chemie/Biologie/Geowissenschaften der Universität Bayreuth genehmigten Dissertation zur Erlangung des Grades eines Doktors der Naturwissenschaften (Dr. rer. nat.). Tag der Einreichung: 20 June 2012 Tag der wissenschaftlichen Kolloquiums: 22 October 2012 Amtierende Dekanin: Prof. Dr. Beate Lohnert Prüfungsausschuß: Prof. Dr. Tomoo Katsura, Universität Bayreuth (Vorsitzender) Prof. Dr. Leonid Dubrovinsky, Universität Bayreuth (Erstgutachter) Prof. Dr. Alexei Bosak, ESRF (Zweitgutachter) Prof. Dr. S. Kümmel, Universität Bayreuth Prof. Dr. J. Senker, Universität Bayreuth Table!of!Contents! ! ! 3! Table&of&Contents& ZUSAMMENFASSUNG5SUMMARY91.-INTRODUCTION-121.1!ORIGIN!AND!THE!STRUCTURE!OF!THE!EARTH!!12! 1.2!MINERALOGICAL!MODEL!OF!THE!MANTLE.!(MG,FE)(SI,AL)O3!PEROVSKITE!AS!DOMINANT! COMPONENT!OF!THE!EARTH!LOWER!MANTLE!!16! - 2.-MOTIVATION-222.1!SPIN!STATE!OF!IRON!IN!THE!LOWER!MANTLE!SILICATE!PEROVSKITE!22! 2.2!NEED!FOR!ENERGYYDOMAIN!SYNCHROTRON!MÖSSBAUER!SPECTROSCOPY!23! ! 3.-METHODS-AND-INSTRUMENTATION-253.1!CREATION!OF!HIGH!PRESSURE!25! 3.2!MÖSSBAUER!SPECTROSCOPY!27! ! 4.-SYNOPSIS-(SCOPE-OF-THE-THESIS)-324.1!THEORETICAL!CALCULATION!AND!EXPERIMENTAL!STUDY!OF!ANGULAR!DEPENDENCES!AND! ENERGY,!TEMPORAL!DISTRIBUTIONS!OF!SYNCHROTRONYBASED!57FE!MÖSSBAUER!RADIATION!32! 4.2!THE!57FE!SYNCHROTRON!MÖSSBAUER!SOURCE!37! 4.3!SPIN!STATE!OF!FE3+!IN!THE!LOWER!MANTLE!39! 4.4!DOUBLE!SIDED!LASER!SYSTEM!FOR!SMS!AND!XYRAY!DIFFRACTION!42! 4.5!LIST!OF!MANUSCRIPTS!AND!STATEMENT!OF!AUTHOR’S!CONTRIBUTION!43! MANUSCRIPTS-465.1-MULTISPACE-QUANTUM-INTERFERENCE-IN-57FE-SYNCHROTRON-MÖSSBAUER-SOURCE--46- ! 5.1.1!ABSTRACT!!46! 5.1.2!INTRODUCTION!!47! 5.1.3!MULTISPACE!INTERFERENCE!OF!γ!RAY!PHOTON!IN!THE!UNIT!CELL!OF!57FEBO3!CRYSTAL.!!50! 5.1.4!NUCLEAR!EXCITON!POLARITON!UNDER!DIFFRACTION!CONDITIONS!!60! 5.1.5!ANGULAR,!ENERGY!AND!TIME!DISTRIBUTIONS!OF!FE!SM!RADIATION!!65! 5.1.6!SUMMARY!AND!CONCLUSION!!73! ! 5.2-ANGULAR-DEPENDENCIES-OF-SPECTRAL-AND-TEMPORAL-DISTRIBUTIONS-OF-NUCLEARRESONANCE-RADIATION-OF-SYNCHROTRON'BASED-57FE-MÖSSBAUER-SOURCE--76- - 5.2.1!ABSTRACT!!76! 5.2.2!INTRODUCTION!!77! 5.2.3.!EXPERIMENTAL!METHODS!!79! 5.2.4.!RESULTS!AND!DISCUSSION!!81! 5.2.5!SUMMARY!!85! Table!of!Contents! ! ! 4! ! 5.3-THE-57FE-SYNCHROTRON-MÖSSBAUER-SOURCE-AT-THE-ESRF--87! ! 5.3.1!ABSTRACT-!87! 5.3.2!INTRODUCTION!!88! 5.3.3!SYNCHROTRON!MÖSSBAUER!SOURCE-!91! 5.3.4!OPTICAL!SCHEME-!102! 5.3.5!PROPERTIES!OF!THE!SMS!RADIATION-!106! 5.3.6!APPLICATIONS-!108! 5.3.7!CONCLUSIONS!!111! 5.4-NO-SPIN-TRANSITION-OF-FERRIC-IRON-IN-THE-LOWER-MANTLE-112- ! 5.4.1!ABSTRACT-!112! 5.4.2!INTRODUCTION!!112! 5.4.3.!EXPERIMENTAL!METHODS-!113! 5.4.4.!RESULTS!AND!DISCUSSION!!114! 5.4.5.!SUPPLEMENTARY!INFORMATION!!119! ! 5.5-PORTABLE-DOUBLE'SIDED-LASER'HEATING-SYSTEM-FOR-ENERGY'DOMAIN-MÖSSBAUERSPECTROSCOPY-AT-SYNCHROTRON-AND-SINGLE-CRYSTAL-DIFFRACTION-EXPERIMENTS-WITHDIAMOND-ANVIL-CELLS--128- ! 5.5.1!ABSTRACT-!128! 5.5.2!INTRODUCTION!!128! 5.5.3.!DESIGN!OF!THE!LASERYHEATING!SYSTEM!!130! 5.5.4.!EXAMPLES!OF!APPLICATION!OF!THE!PORTABLE!LASERYHEATING!SYSTEM!!135! 5.5.5.!CONCLUSIONS!!!139! ! 5.6-FULL-LIST-OF-PUBLICATIONS-(WITH-PAPERS-NOT-INCLUDED-INTO-THE-THESIS-)--141- ! BIBLIOGRPHY-142! ! ACKNOWELEGMENTS-156ERKLÄRUNG-157 Zusammenfassung! ! ! 5! Zusammenfassung Eisen ist eines der häufigsten Elemente der Erde und eine wichtige Komponente in Mineralien. Elektrische und magnetische Eigenschaften eisenhaltiger Materialien beeinflussen signifikant Prozesse in der Tiefe der Erde. In den Materialien, die den Unteren Mantel ausmachen, kann Eisen sowohl in verschiedenen Valenzund Spinzuständen als auch in verschiedenen kristallographischen Umgebungen vorkommen. Die meisten experimentellen Techniken gestatten es nicht, die Entwicklung der Eigenschaften verschiedener Eisenplätze individuell zu verfolgen oder sie sind nicht geeignet für Untersuchungen unter gleichzeitigen Hochdruckund Hochtemperaturbedingungen. Unter diesen Bedingungen werden Untersuchungen zur elektronischen Struktur von Eisen eine große Herausforderung. Die vorliegende Doktorarbeit ist in zwei Hauptabschnitte gegliedert. Der erste Teil beschäftigt sich mit der Entwicklung einer Synchrotron Mössbauer Quelle (Synchrotron Mössbauer Source, SMS). Sie erlaubt eine energieabhängige Mössbauerspektroskopie von Proben unter Drücken jenseits von 100 GPa in lasergeheizten Diamanthochdruckzellen. Der zweite Teil der Arbeit ist dem Verhalten von Eisen in eisen-/aluminiumhaltigen Silikat-Perowskiten unter Bedingungen des Unteren Mantels gewidmet. 1. Synchrotron Mössbauer Quelle Es existieren mehrere Techniken, um elektrische und magnetische Eigenschaften von Materialien unter extremen Bedingungen zu untersuchen: Röntgenemissionsspektroskopie (XES), Nahkanten-Röntgenabsorptionsspektroskopie (XANES), Kernresonazstreuung, usw. Die ausgereifteste, empfindlichste und geeigneteste Technik für solche Studien ist jedoch die (energiedifferenzielle) Mössbauerspektroskopie. Die niedrige Brillanz der dazu benötigten radioaktiven Quellen und die niedrige Anreicherung des Eisens in den Mineralien des Unteren Mantels erfordert jedoch im Falle der energieaufgelösten Mössbauerspektroskopie lange Messzeiten und limitiert die Anwendung auf moderate Drücke. Dieser Umstand würde durch die Kombination der ausergewöhnlichen Eigenschaften der Synchrotronstrahlung (hohe Brillanz, Möglichkeiten zur extremen Fokussierung) mit denen der klassischen Mössbauerspektroskopie gelöst werden. In kurz: Es wird eine Synchrotronquelle mit Zusammenfassung! ! ! 6! Mössbauerstrahlung benötigt. Die Konstruktion einer solchen Quelle war die Hauptaufgabe meiner Promotionsarbeit. Smirnov et al. (1997) hat die Möglichkeit einer solchen Quelle an der Nuclear Resonance Beamline (ID18) der European Synchrotron Facility (ESRF) demonstriert. Die Quelle basiert auf reinen Kernreflexen im antiferromagnetischen Kristall FeBO3. Die technischen Hauptaufgaben meiner Promotionsarbeit waren (a) die Konstruktion einer permanent verfügbaren Quelle mit (b) der bestmöglichen Energieauflösung bei gleichzeitiger höchstmöglicher Intensität. Um diese Ziele zu erreichen, wurden zunächst theoretische Rechnungen durchgeführt, um die beste Strategie zur Optimierung der Quelle zu finden. Mehrere Experimente folgten, um die theoretischen Ergebnisse zu verifizieren. Zudem wurden mehrere optische Anordnungen der SMS getestet, um die optimale Anordnung zu bestimmen. Das Ergebnis des wissenschaftlichen Entwicklungsprogramms ist eine wirkungsvolle Synchrotron Mössbauer Quelle für Hochdruckanwendungen an der Nuclear Resonance Beamline (ID18) der ESRF. Theoretische und experimentelle Ergebnisse zur Winkelabhängigkeit der Energieund Zeitverteilung der reinen Kernreflexe des Eisenboratkristalles erlaubten es, die SMS bezüglich höchstmöglicher Intensität und bestmöglicher Energieauflösung zu optimieren. Die Energiebandbreite der Strahlung der SMS ist etwa 15 neV (3 Γ0), die Intensität etwa 2,5·10 4 Photonen/s und der typische durchstimmbare Geschwindigkeitsbereich etwa ± 12 mm/s (± 0,6 µeV). Im Gegensatz zu klassischen radioaktiven Quellen kann die Strahlung der SMS in den Bereich von zehn Mikrometern fokussiert werden. Die SMS ist ein 'in-line' Monochromator, der permanent in der optischen Hütte der Beamline installiert ist und nach Einbringung in den Synchrotronstrahl betriebsbereit ist. Sie kann in Verbindung mit allen verfügbaren Probenumgebungen der Beamline betrieben werden. Die Verfügbarkeit der SMS eröffnet Möglichkeiten, Systeme mit komplexen Hyperfeinstrukturen unter extremen Bedingungen zu untersuchen, wie z.B. unter Ultrahochvakuum und Hochdruck. Weiterhin erlaubt es die Quelle, Spektren in sehr kurzer Zeit, d.h. in wenigen Minuten, zu messen; kurz genug, um Daten auch in Verbindung mit Laserheizung zu nehmen. Mehrere Hochdruckmessungen und kombinierte Hochdruck-, Hochtemperaturmessungen wurden im Rahmen der Doktorarbeit durchgeführt. Diese werden im zweiten Teil der Doktorarbeit beschrieben. Die fast 100-prozentige rückstoßfreie Strahlung der Quelle und ihre hohe Zusammenfassung! ! ! 7! Brillanz erlaubt viele SMS Anwendungen. Die SMS kann in allen Betriebsmoden eines Synchrotrons betrieben werden. 2. Untersuchungen zum Spinzustand der Fe3+ Ionen in Perowskiten Silikat-Perowskit (Mg,Fe)(Si,Al)O3 ist die am häufigsten vorkommende Phase im Unteren Mantel der Erde. Die Kenntnis ihrer Eigenschaften ist unabdingbar für das Verständnis des Verhaltens des Unteren Mantels. Dynamische, thermodynamische und Transporteigenschaften der Silikat-Perowskite können empfindlich durch den Valenzund Spinzustand des Eisens beeinflusst werden. Silikat-Perowskite mit 5-10 mol% Eisen (Fe3+ / ΣFe ~ 50-75%; McCammon et al., 1997) und Al ist die dominante Phase im Unteren Mantel der Erde (~ 75 vol%)(Zhang et al., 2006; Stackhouse et al., 2007). Dennoch bleiben die Veränderungen der elektronischen Eigenschaften des Eisens unter ähnlichen Bedingungen wie des Unteren Mantels weiterhin umstritten. Der zweite Teil meiner Promotionsarbeit ist den Untersuchungen des Spinzustandes in Fe3+ reichen Silikat-Perowskiten unter Hochdruck gewidmet. Vier verschiedene Silikat-Perowskitproben mit unterschiedlicher Stöchiometrie wurden mit Hilfe der Synchrotron Mössbauer Quelle untersucht. Die SMS Spektren wurden bei Raumtemperatur und unter Drücken bis 122 GPa in einer Diamant-Stempelzelle gemessen. Es wurden Proben wie hergestellt bzw. auch thermisch ausgeheilte (mittels Laserheizung) benutzt. Die aus den Messungen gewonnenen Hyperfeinwechselwirkungsparameter, d.h., die Isomerieverschiebung und die Quadrupolaufspaltung, sind bei gleichem Druck für alle Proben dieser Arbeit innerhalb des experimentellen Fehlers gleich. Zudem ändert sich das Fe3+/ΣFe Verhältnis nicht für die individuellen Proben über den gesamten Druckbereich des Experiments. Die Hyperfeinwechselwirkungsparameter des Fe3+-Dubletts entsprechen einem Hochspinzustand (Gütlich et al., 2011) und ihre flache Abhängigkeit vom Druck deutet an, dass Fe3+ keinen Spinübergang im gesamten Druckbereich macht. Alle beobachteten Änderungen in den Spektren sind den Änderungen des elektronischen Zustandes des Fe2+ zuzuschreiben. Die Hyperfeinwechselwirkungsparameter des Fe2+- Dubletts mit der kleinen Quadrupolaufspaltung entsprechen einem Hochspinzustand (McCammon et al., 2008), während das Dublett mit der grossen Quadrupolaufspaltung, dessen Anteil mit zunehmendem Druck auf Kosten des Zusammenfassung! ! ! 8! Hochspinzustandes zunimmt, entweder einem Zwischenspinzustand (IS) des Fe2+ entspricht (McCammon et al., 2008) oder mit einer Verzerrung der Umgebung des Platzes des Hochspin-Fe2+ erklärt werden kann (Hsu et al., 2010). Ungeachtet der Interpretation des Fe2+ Spinzustandes bleibt die Schlussfolgerung bezüglich des nicht vorhandenen Spinüberganges in Fe3+ dieselbe. Diese Ergebnisse zeigen, dass der kürzlich berichtete Spinübergang in Fe3+ nicht stattfindet, wenn Fe3+ Ionen die A-Plätze besetzen. In diesem Fall bleiben die Fe3+ Ionen in Silikat-Perowskiten sowohl mit als auch ohne Aluminium bis midestens 122 GPa im Hochspinzustand. Dieser Druck entspricht fast den Bedingungen, die an der Grenze zwischen dem Unteren Mantel und dem Äusseren Kern herrschen. Die Ergebnisse zeigen auch, dass Fe3+ Ionen unter Hochtemperaturausheilung und Hochdruck nicht von den A-Plätzen in die B-Plätze diffundieren. Demzufolge gibt es auch kein Anzeichen für einen Hochspinzu Niederspinübergang auf Grund eines Platzwechsels der Fe3+ Ionen. Dies steht im Gegensatz zu den Fe2+ Ionen, die von einem Hochspinin einen Zwischenspinzustand übergehen, ohne dabei einen Niederspinzustand bei Raumtemperatur in dem untersuchten Druckbereich zu erreichen. Diese Ergebnisse lassen vermuten, dass die seismischen Geschwindigkeitsanomalien im Unteren Mantel nicht einem Spinübergang in Fe3+ zuzuschreiben sind. Summary! ! ! 9! Summary Iron is one of the most abundant elements on Earth, and it is an important component in minerals. Electronic and magnetic properties of iron-bearing materials significantly affect processes occurring in the deep interior of the Earth. In the materials that make up the Earth’s lower mantle iron may exist in different valence, spin states and crystallographic environments. Most of the existing experimental techniques either do not allow to separately follow evolution of different iron sites or are not suitable for measurements under high-pressure/high-temperature conditions. This makes studies of iron electronic structure under such conditions extremely challenging. The current Ph.D. thesis is divided into two major parts. The first part is dedicated to the development of a Synchrotron Mössbauer Source (SMS). This device allows energy domain Mossbauer spectroscopy to be performed on a sample under pressures above 100 GPa in laser heated diamond anvil cells. The second part is dedicated to studying the behavior of iron in iron/alumina-bearing silicate perovskite under conditions of the Earth’s lower mantle. 1. Synchrotron Mössbauer Source There are several techniques that allow magnetic and electronic properties of materials under extreme conditions to be probed: X-ray Emission Spectroscopy (XES), X-ray absorption near edge structure (XANES), Nuclear Resonance Spectroscopes, etc. For elements in which observation of Mössbauer effect is possible the most mature, sensitive, and suitable technique for studies of magnetic and electronic properties is energy-domain Mössbauer spectroscopy. However, due to low brilliance of utilized radioactive sources and low natural abundance of iron in lower mantle minerals measurements using conventional energyresolved Mössbauer spectroscopy require very long time and usually are limited to moderate pressures. The problem can be solved by combining the outstanding properties of synchrotron radiation (high brilliance, possibility for extreme focusing) with the energy-resolved approach. In brief, what is needed is a synchrotron source of Mössbauer radiation. Construction of such source was the primary task of my PhD work. The possibility to develop such a source was demonstrated at the Nuclear Resonance beamline ID18 at the European Synchrotron Radiation Facility (ESRF) by Smirnov et al. (1997). The source is based on pure nuclear reflections existing in antiferromagnetic 57FeBO3 crystals. 1.!Introduction!! ! ! ! ! 16! constrained by both its bulk chemical composition (Table 1.1) and seismic profiles, providing the most detailed information regarding the structure of the Earth’s interior. Analyses of seismic waves profiles revealed the presence of the two major discontinuities within the mantle: (I) the so-called transition zone at a depth from 410 to 660 km, which defines the boundary between the upper and lower mantle; (II) and the D’’ layer that lies just above the core-mantle boundary (Fig. 1.1). The origin of the mantle discontinuities will be discussed in the following section. 1.2. Mineralogical model of the Mantle. (Mg,Fe)(Si,Al)O3 perovskite as dominant component of Earth lower mantle. Even through element composition of mantle is quite homogeneous, the structure of minerals from which is composed gradually change with pressure. At the small depths (<100 km) Earth composed of 4 major phases: olivine, orthopyroxene, clinopyroxene and garnet. In the framework of the popular “pyrolite model” (Ringwood, 1975), it is believed that the (Mg,Fe)2SiO4 olivine composes about 60% of the Earth’s upper mantle(Fig. 2) and that the nature of mantle heterogeneities are related to the depth-varying behavior of olivine. The first transition happens at the upper boundary of the transition zone (the layer in the lowermost upper mantle, characterized by anomalous behavior of seismic wave velocities) where (Mg,Fe)2SiO4 olivine transforms to its high pressure polymorph – β-phase or wadsleyite with a modified spinel structure, which occurs at around 14 GPa (410-km depth) (Fig. 1.2). Then at about 17 GPa (500-550 km depth) wadsleyite transforms to a more densely packed spinel-structured phase – (Mg,Fe)2SiO4 ringwoodite. In turn, ringwoodite disproportionates to (Mg,Fe)SiO3 perovskite and (Mg,Fe)O ferropericlase. This disproportion defines the lower mantle composition and the bottom of transition zone, which is the 660-km discontinuity. 1.!Introduction!! ! ! ! ! 17! Figure 1.2 Mineral volume fraction of the pyrolite model mantle (Ringwood 1989, 1991; Stixrude and LithgowBertelloni 2005). Opx and cpx are acronyms for orthopyroxene and clinopyroxene respectively. Modified after Frost, (2008). The remaining non-olivine components (together they make ~40% of upper mantle volume): orthopyroxene, clinopyroxene and garnet, undergo gradual transitions at the depth from 350 to 450 km: pyroxene dissolves in garnet via MgSi → Al2 substitution, resulting in the formation of Al-depleted garnet (majorite) at pressures above 16 GPa (Irifune and Ringwood, 1987), which eventually also transforms to silicate perovskite (at 23 – 26 GPa or 660 - 750 km depth) with exsolution of Ca-bearing silicate perovskite (Fig. 1.2). Since the dissolution of pyroxenes in garnet is “smeared out” over more than 100 km, it leads to changes in slope of the curves of seismic velocity versus depth rather than causing discrete discontinuities. 1.!Introduction!! ! ! ! ! 18! Figure 1.3 Polyhedral models of perovskite (Ross and Hazen, 1989). Big solid grey spheres correspond to Mg2+/Fe2+/3+ in tetradecahedral site; Si4+/Al3+ octahedra in the perovskite structure are shown in green. Summarizing all above, in the lower mantle we have three dominant phases: (Fe,Mg)O ferropericlase, MgSiO3 silicate perovskite and CaSiO3 perovskite. After majoritic garnet (the main reservoir of Al in upper mantle), becomes unstable, the perovskite is the main candidate for a phase in lower mantle, which can incorporate Al. Iron in the lower mantle is divided between ferropericlase and silicate perovskite. Finally, at pressures and temperatures close to those of the core-mantle boundary magnesium silicate perovskite (space group Pnma) undergo a structural transition into the post-perovskite phase (space group Cmcm) (Murakami et al., 2004). This transition is believed to be “responsible” for the D’’ seismic discontinuity at 2600-km depth (Fig. 1.1 and Fig. 1.2). The iron containing silicate perovskite is a dominant phase in the Earth lower mantle, making ~80% of its volume (almost ~ 48% of entire volume of the Earth). Therefore, this perovskite generally defines lower mantle seismic and density profiles, electrical and thermal conductivity, redox state, etc. Thus understanding of the silicate perovskite properties is critical for construction model of the Earth. 1.!Introduction!! ! ! ! ! 19! The ideal cubic ABX3 perovskite crystal structure with space group Pm3m consists of a three-dimensional network of the corner connected octahedra (B-site) and the cavities between the octahedra form the tetradecahedral site (A-site). The lower mantle silicate perovskite has the GdFeO3 structure type, which is distorted from the ideal cubic symmetry by tilting of the octahedra and has the space group Pnma (Horiuchi et al., 1987). It has two sites: the large distorted 8to 12fold coordinated A-site predominantly occupied by a Mg2+ (big solid grey spheres in Fig. 1.3) (Horiuchi et al., 1987, McCammon et al., 1992); and the smaller nearly symmetrical octahedral B-site predominantly occupied by a Si4+ (small colored in green in Fig. 1.3). Let’s look in the more details; so the base composition for the lower mantle perovskite is the MgSiO3 (MgPv), where Mg2+ and Si4+ cations are balanced by O2anions. In the lower mantle, it is assumed that the MgPv incorporate Fe (in two states Fe3+ and Fe2+) and non-negligible amounts of aluminum. Let’s look in the more details how this cations can enter perovskite. The aluminum ions Al3+ due to their small ionic radius (0.53 pm) can enter only the B-site of the silicate perovskite through the Si4+ substitution. However, such substitution creates the imbalance in the charge of the system. So it should be coupled with creating of an oxygen vacancy, otherwise a 3+ ion should substitute the Mg2+ in the A-site. In case of Fe3+ the situation is more complicated. Its ionic radius of 0.63 pm is closer to that of silicon than ionic radius of ferric ions. Thus it was proposed that ferrous iron could enter B-site as well as A-site of MgPv and substitute both Mg2+ and Si4+. The Fe3+ ion, which enters the A-site, allows the Al3+ to enter the B-site without creating oxygen defects in the MgPv. The Fe2+ ions can enter the A-site of MgPv substitute the Mg2+ one to one. On the other hand, ferrous iron can’t enter Bsite due to fact that Fe2+ ionic radius of 0.77 pm is bigger than 0.4 pm in case of Si4+ and because of needs to balance charge of the of system. Several mechanisms for such substitution were proposed, namely, Mg2+A + Si4+B to Fe3+A + Fe3+B, Mg2+A + Si4+B to Fe3+A + Si4+B+□2- (oxygen vacancy) and Mg2+A + Si4+B to Fe3+A + Al3+B; where A and B subscripts indicate the corresponding MgPv site. The last mechanism attracts increasing attention as; silicate perovskite with 5-10 mol% of Fe and Al is dominant 1.!Introduction!! ! ! ! ! 20! phase in Earth lower mantle (~75 vol%) (Zhang et al., 2006; Stackhouse et al., 2007) and it allows for both cations to enter MgPv without creating of the oxygen vacancies which a not energetically favorable under high-pressure. It was found, that the silicate perovskite in its stability field has considerable amount of the Fe3+, up to Fe3+/ΣFe ~50–75% (McCammon et al., 1997). Enriching of perovskite in Fe3+ is believed to be due to the crystal chemistry; in presence of the aluminum iron predominantly is in Fe3+ state (McCammon et al., 1997) and not due to oxidation state of the lower mantle (Frost et al., 2004; McCammon et al., 2005). Therefore, the iron/aluminum containing silicate perovskite (FeAlPv) is the most appropriate system to study a role of iron in the lower mantle properties. Behavior of the iron is different for its various valence state and depends on which crystallographic site it occupies in MgPv. There is no agreement in the pressures of the spin transition of the Fe3+ reported by previous studies, broadly a high-spin (HS) (five unpaired d electrons) to low-spin (LS) (one unpaired d electron) transition is predicted when Fe3+ occupies the B-site, while Fe3+A is predicted to remain in the high-spin state at all pressures throughout the lower mantle (Zhang et al., 2006; Stackhouse et al., 2007; Catalli et al., 2010 and 2011; Hsu et al., 2011). Even though Fe3+ is predicted to occupy exclusively the A-site in lower mantle compositions of FeAlPv (Vanpeteghem et al., 2006), a HS-LS transition of Fe3+ was reported to occur in FeAlPv at high pressure, potentially due to exchange of Fe3+ from the Ato the B-site (Catalli et al., 2011; Hsu et al., 2011). Iron is playing an essential role in the modeling of the lower mantle properties. This transition element influences chemical partitioning and reactions among mantle minerals, as well as thermal, electrical, and mechanical properties at depth (Fyfe, 1960; Burns, 1970; Gaffney and Anderson, 1973; Sherman, 1988; Lin et al., 2005, 2006; Goncharov et al., 2006; Keppler et al., 2007). For example, it was found that presence of the Fe3+ could control the radiative conductivity of the perovskite in the lower mantle (Keppler et al., 2007). Already more than 50 years ago, Fyfe (1960) predicted that a spin-pairing transition of iron takes place within the lower mantle. Indeed the transition was experimentally observed in the ferropericlase (Badro et al., 2003; Lin et al., 2005, 2006; Speziale et al., 2005; Kantor et al., 2006a) 1.!Introduction!! ! ! ! ! 21! and the silicate perovskite (Badro et al., 2004; Jackson et al., 2005; Li et al., 2006). It was found out that these transitions strongly affect an elastic properties of the ferropericlase; for example change of the shear velocities reaches ~4% (Murakami et al., 2012). This indicates that existence of the spin transition of the iron in the MgPv may lead to the change in it share velocities. Thus, existence of a so-called “Spin transition zone” in the lower mantle could explain phenomena like, for example, a small seismic heterogeneities (Hedlin et al., 1997; Kaneshima et al., 1999). The essence of these heterogeneities can also be related to a small fluctuations in a Mg/Fe or a (Mg,Fe)/Si ratios (Bina, 2003). Such a fluctuation can be, in turn, caused by the spin transitions in the lower mantle assemblage (Lin et al., 2007). The Earth mineralogy model is constrained through comparison of sound velocities obtained in laboratory studies to the seismological data. Recently Murakami et al., (2012) had measured Vs for iron-free MgPv containing Al and compared results to Preliminary Reference Earth Model (PREM). They came to conclusion that the lower mantle on at least 93% consists of the MgPv. So, in the lower mantle a Mg/Si ratio is around 1. Such a high proportion is more consistent with the chondritic mantle model than with conventionally used peridotitic mantle model. Such, chemical stratification implies layered mantle convection with limited mass transport. However, in that study iron free MgPv was studied, such composition does not correspond to the real life composition of the Earth lower mantle. As was stated before the spin transition of the iron in the MgPv could lead to change in the Vs values for it. Thus, determination of the iron state is critical for determination of the correct mantle model, which, in turn would lead to construction of the correct model for the convection in the Earth mantle. 2.!Motivation!! ! ! ! ! 22! 2. Motivation 2.1 Spin state of iron in the lower mantle silicate perovskite Lower mantle make up to 60% of Earth volume. This makes lower mantle the largest (by volume) entity of the Earth. Thus, it is impossible to construct a reliable model of the Earth without proper understanding of lower mantle properties. It is believed that Earth lower mantle incorporates in average ~10 mol.% of iron in form of Fe2+ and Fe3+ (Sturhahn et al. 2005). Iron is incorporated in two major phases of lower mantle ( >90% of it volume, Fig. 1.2): ferropericlase and Al-bearing magnesium silicate perovskite. Thermal and electrical conductivity, and elastic properties of the lower mantle may strongly depend on iron oxidation and electronic state (Xu et al. 1998, Lin et al. 2005, Kepler et al. 2008). Thus, iron is probably the most important transition element in the physics of the Earth. Construction of reliable model of the Earth is impossible without constraining the evolution of iron properties in ferropericlase and silicate perovskite under lower mantle conditions. The behavior of iron in ferropericlase is well constrained (Kantor et al, 2006, Murakami et al., 2012). On the other hand, reports on behavior of iron in silicate perovskite remain strongly controversial. There are three main reasons to that. First, iron exists in perovskite in two valence states simultaneously (2+ and 3+ valence states). Second, the structure of magnesium silicate perovskite contains two sites, large 8+4 coordination site (“A”) occupied by Mg2+ and a smaller octahedral site (“B”) occupied by Si4+. Even through iron expected to reside in the A-site, there are reports, which suggest that Fe3+ ions can also enter the B-site (Catalli et al. 2010). Third, all iron electronic states and crystallographic environment may depend on pressure and/or temperature. All together make that irons bearing magnesium silicate perovskite extremely challenging system to study spin state of iron in it. Effect of Al on Fe state should be mentioned here as well. There are several different scenarios describing the spin transition of iron in silicate perovskite: 1) Both Fe3+ and Fe2+ ions undergo a high-spin to low-spin transition (Badro et al. 2004). 2.!Motivation!! ! ! ! ! 23! 2) Only Fe3+ ions located in A-site undergo high spin to low-spin transition (Jackson et al. 2005, Zhang et al. 2006, Stackhouse et al. 2007). 3) Fe2+ ions undergo high-spin to intermediate spin transition, while Fe3+ stay in high-spin state (McCammon et al. 2008, Narygina et al. 2010, Lin et al. 2008). 4) Only Fe3+ ions located in B-site undergo high spin to low spin transition (Catalli et al. 2010). 5) Fe2+ ions stay in high spin state in lower mantle (Zhang et al. 2006, Cohen et al. 1997, Li et al. 2005), while observed changes in electronic structure of Fe2+ ions are explained by crystallographic distortions of the A-site (Lin et al. 2012). 6) That a Fe3+ ions at high-pressure and high-temperature “diffuse” to B-site at the same moment as undergo high spin to low spin transition (Catalli et al., 2011). There is no agreement on which of these models describes the behavior of iron in the lower mantle. We have performed the in situ measurements of iron hyperfine parameters in iron/alumina bearing silicate perovskite samples with different stoichiometry in a laser-heated diamond anvil cells (DACs) in order to investigate the spin state of Fe2+ and Fe3+ at conditions of Earth lower mantle, namely, under pressure up to ~ 130 GPa and temperature up to ~ 3500 Co. In order to perform such study and achieve this goal we developed new experimental tool that was set up at ID18 at ESRF – Synchrotron Mössbauer Source, as usually used methods are not very suitable for this kind of experiments. 2.2 Need for energy-domain synchrotron Mössbauer spectroscopy Behavior of the iron in silicate perovskites under high-pressure can be investigated by many different techniques: X-ray Emission Spectroscopy (XES), Nuclear Forward Scattering (NFS), X-ray absorption near edge structure (XANES) and related X-ray Magnetic Circular Dichroism (XMCD), conventional Mössbauer spectroscopy and X-ray powder and single crystal diffraction (XRD). Unfortunately application of any of these powerful techniques does not allow unambiguous 2.!Motivation!! ! ! ! ! 24! determination of the spin state of iron in lower mantle perovskite. For example, although XANES is sensitive to ions valence state and XMCD gives an estimate of <Lz> and <Sz>, there are not well suited for the cases when the same element can be found in different valence state and crystallographic sites (like iron in silicate perovskite). XRD is not a direct method for testing of spin state. XES provides information on the bulk on density of electronic states. This makes separation of the individual contributions from ions with different oxidation state and/or crystallographic sites extremely challenging. On the other hand energy domain 57Fe Mössbauer spectroscopy in many cases enables an unambiguous resolution of all hyperfine parameters of ions in different oxidation and electronic states and different crystallographic environment. High-pressure measurements using conventional radioactive point sources encounter several difficulties such as long counting time (generally more than one week per spectrum at ~50 GPa pressure range), difficulties to focus the beam, and strong background. All these difficulties greatly reduce the quality of the spectra and thus resolution and sensitivity of the technique. The third generation synchrotron facility offers a solution in the form of time-domain counterpart of conventional Mössbauer spectroscopy (i.e., nuclear forward spectroscopy, NFS). However NFS is not well suited for high-complexity systems, which contain iron nuclei in different spin, valence states and crystallographic sites. In order to study spin transition in perovskite under lower mantle conditions we have developed an energy-domain synchrotron Mössbauer source. New methodology allows performing of energy domain Mössbauer measurements using synchrotron as a source of gamma radiation. SMS inherits all of the advantages of the conventional Mössbauer spectroscopy and on top of that offers a number of new benefits: high flux, a beam diameter of a few microns and zero background. SMS allows for rapid measurement of energy-domain Mössbauer spectra under extreme conditions with a quality generally sufficient to unambiguously deconvolute even highly complex spectra. These features were successfully demonstrated in our investigation of iron behavior in Al-bearing silicate perovskite under high-pressure. 3.!Methods!and!Instrumentation!! ! ! ! ! 25! 3. Methods and Instrumentation 3.1. Creation of high pressure The first experiments with diamond anvil cells were performed in late 50s of the 20th century (Weir et al. 1959). The method has started to gain popularity only after a discovery of a reliable pressure standard in the beginning of 70s (Forman et al. 1972). Since the technique appears it has been constantly evolving, and the number of possible applications increases. Now the list of DAC applications includes measurements of magnetic properties, resistivity, nuclear magnetic resonance, x-ray diffraction, x-ray absorption, optical spectroscopy, conventional Mössbauer spectroscopy, Nuclear Forward Scattering etc. (Chervin et al. 1995, Mito et al. 2001, Garg et al. 2004, Haase et al. 2009). Operation of diamond anvil cell is very simple and can be described as below. The sample chamber is pressurized by application of force on two uni-axially aligned diamonds, this leads to compression of a sample, a pressure transmitting medium and a pressure standard. The force can be applied in various ways. One of the most popular ways is tightening of the screws (Merrill and Bassett 1974) another one is rising pressure in a membrane for the membrane cell (Chevrin et al., 1995). Basic diamond anvil cell (modified after Merrill-Basset design) is shown in Figure 3.1. In our experiments, two types of DACs were used; they are shown in Figure 3.2. It is the four pin modified Merrill-Basset cell and the piston-cylinder cell, developed in Bayerisches Geoinstitut (BGI). The typical diameter for these cells is Ø50mm and the average height is between 40 and 45 mm. The upper limit of pressure generated by diamonds depends on two factors, first-on the size of the diamonds culets and second-on the design of diamonds. In practice the simple rule is used: the smaller is diamonds culet size – the higher maximum pressure can be reached. In the works described below, usually diamonds with a culet diameter Ø250-300 µm were employed. To generate high-pressure without breaking of the diamond anvils several things are important: diamond supporting plates (seats), correct material of the gasket and good mechanical stability of the diamond anvil cell. Design of diamond backing plates depends on experimental technique and it manufacturer. In BGI DACs used for experiment hardened tungsten carbide plates were employed, without any specific 4.!Synopsis!(Scope!of!the!Thesis)! ! ! ! ! 32! 4. Synopsis (Scope of the Thesis) This chapter provides a summary of the results presented in five papers comprising the Chapter 5. This chapter is organized in following way: Subsection 4.1 is dedicated to theoretical and experimental investigation of angular, energy and temporal properties of pure nuclear reflection of iron borate crystals near its Néel temperature. In subsection 4.2 we present a description of SMS developing and examples of its application beyond high-pressure physics. Subsection 4.3 contains report on spin state of Fe3+ ions in perovskite under lower mantle conditions. Finally, in section 4.4 combination of SMS with portable double-sided laser system is described. This combination allows us to perform energy-domain Mössbauer measurements under high-pressure and high-temperature extreme conditions simultaneously. I wrote three out of five papers presented in Chapter 5 as a first author. For them I had prepared and conducted all experiments, analyzed the data, and performed calculations. I had developed the Synchrotron Mössbauer Source in close collaboration with other coauthors. I had also written the text of the publications (alone or with advices other co-authors) In percents my contribution to Chapters 5.2 and 5.3 is 80% and to 5.4 is 75%. Two manuscripts (Sections 5.1, 5.5) were not written by me as a first author. For the paper described in Section 5.1 I had participate in calculations, data analysis, and text preparations. My total contribution to the paper is around 45%. For fifth paper described in Chapter 5.5 I have prepared and conducted laser-heating experiments with SMS, acquired and analyzed measured spectra, and contributed to the paper preparation, which make my contribution on the level of 33%. 4.1 Theoretical calculation and experimental study of angular dependence, energy and temporal distributions of synchrotron-based 57Fe Mössbauer radiation We had performed a theoretical calculation of the angular dependences and energy, temporal distributions for pure nuclear reflections in crystal of iron borate 4.!Synopsis!(Scope!of!the!Thesis)! ! ! ! ! 33! (57FeBO3) at various values of internal magnetic field. These calculations were performed in order to look at the behavior of these distributions while internal magnetic field approaching it collapse, which happens at Néel point. It was found that following the collapse of the internal magnetic field angular dependence of the reflected radiation (rocking curve) becomes significantly broader and change its shape from Gaussian-like to complex structure with two peaks (Fig. 4.1.2-a). On the other hand, energy and time distributions demonstrate strong angular dependence. Only in angular position of the minimum between two peaks (which corresponds to exact value of the Bragg angle) energy distribution consists from just one line (hereafter main line). Outside of this narrow angular region, the main line is accompanied by satellites (Fig. 4.1.1-b). In the region of the lower angle satellite has a lower energy than main line, in the region of higher angles satellite energy is slightly higher. An analysis of time distributions gives (Fig. 4.1.3) an additional insight at the involved processes. The non-exponential decay shape of time distributions in angular position of the minimum reveals a true nature of the main line, which was obtained near the Néel point. In fact this is pseudosingle line formed as the result of the collapse of the hyperfine magnetic structure of nuclear levels. In this particular case, the time distribution follows such dependence: , (4.1.1) where τ – lifetime of the level in the excited state, t – time, I(t) – intensity. The Eq. (4.1.1) actually is a module of the Fourier transform taken from following equitation (in limit of → 0): , (4.1.2) where – amplitude of the scattered wave, – difference in energy between two lines, – width of the line ( € τ ×Γ≈ ). The Eq. (4.1.2) describes interference between two Lorentzian lines with opposite sign. In first approximation, the module taken from of Eq. (4.1.2) at the limit of → 0 describes the main line (which is our pseudosingle line) and in the first approximation followed squared Lorentzian distribution: 4.!Synopsis!(Scope!of!the!Thesis)! ! ! ! ! 34! , (4.1.3) where – energy, – a value of resonance energy, L(E) – intensity. The results of the experiments performed in almost perfect conditions (highquality iron borate crystal, very small divergence of the incident beam etc.) are in very good agreement with theoretical predictions. Indeed in vicinity of Néel temperature, the width of the crystal rocking curve becomes significantly broader and rocking curve changes its shape from Gaussian-like to complicated structure with two peaks (Figure 4.1.2). Furthermore, the angular and time distributions acquired a strong angular dependence (Figure 4.1.3). As predicted, in the angular region in the minimum between two peaks of rocking curve the energy distribution consist of only one main line free of satellites. The full width at half maximum (FWHM) of this line is of ~10 neV (2 ). Analysis of time distribution showed that the distribution obtained in the minimum angular position can be fitted using Eq. 4.1.1 (Figure 4.1.32b). Thus, in first approximation the experimentally obtained main line obeys squared Lorentzian distribution. 4.!Synopsis!(Scope!of!the!Thesis)! ! ! ! ! 35! Figure 4.1.1 Calculated angular, energy and temporal distributions for (333) pure nuclear reflections of iron borate crystal. (a) – Rocking curves calculated with different value of internal magnetic field. Energy and temporal distribution was calculated for value of the external magnetic field equal to 2kOe. The dip zone (taken from Fig.4.1.1a) is shown on the upper panel on Fig. 4.1.1b. The energy and time distributions on the lower panels refer to the left and the right sides of the dip zone, L and R respectively, as shown on the upper panel. The energy and time distributions at the exact dip position are displayed on both L and R panels (bold line curves). Each next distribution corresponds to an angular shift of the crystal from the dip position by 5 μrad.!For better visualization the energy and time distributions are equally spaced along the vertical axis. (b) – Energy distributions at different angular position. Only in the angular position, which corresponds to the exact Bragg angle, main line is practically free from satellites. (c) – Temporal distributions at angular positions at which energy distributions was calculated. 4.!Synopsis!(Scope!of!the!Thesis)! ! ! ! ! 36! Figure 4.1.2 Angular dependence of the reflected radiation for iron borate (333) pure nuclear reflection of IB crystal (a) at RT (b) 348.9 K. Circles show experimental points, solid line shows theoretical fit. On panel b letters (from a to d) mark angular position of the crystal relative to the incident beam in which energy and time distribution was measured. Figure 4.1.3 Measured Energy and time distributions of nuclear Bragg diffraction for iron borate (333) reflection. [1] - Energy distributions of nuclear Bragg diffraction for iron borate (333) reflection measured in different angular position of the crystal relative to the incident beam. Olive circles - experimental points, red line - theoretical fit. Energy distribution was measured using standard Mössbauer K2Mg57Fe(CN)6 single line absorber. [2] - Time distributions of nuclear Bragg diffraction for iron borate (333) reflection measured in different angular position of the crystal relative to the incident beam. Olive circles shows experimental points, red line - theoretical fit, blue dash line shows fit to data using Eq. 4.1.1. Measurements performed at four angular positions (4.1.2b): small peak (a), minimum (b), big peak (c) and the right slope of the big peak (d). 4.!Synopsis!(Scope!of!the!Thesis)! ! ! ! ! 37! 4.2 The 57Fe Synchrotron Mössbauer Source Using results summarized in Section 4.1 (Chapters 5.1 and 5.2) we design a 57Fe Synchrotron Mössbauer Source. It allows to perform energy-domain Mössbauer measurements using synchrotron radiation. The SMS is based on pure nuclear reflection of an iron borate crystal (57FeBO3). Figure 4.2.1 shows the optical scheme for high-pressure experiment with a DAC using the SMS based on (333) pure nuclear reflection. The SMS is the in-line monochromator, i.e., the beam emitted by the source is directed almost exactly along the synchrotron radiation beam from highheatload monochromator. We developed design for two in-line optical schemes based on (111) and (333) pure nuclear reflections of the 57FeBO3. Figure 4.2.1 The optical scheme for a high-pressure experiment with DAC using the Synchrotron Mössbauer Source based on the (333) pure nuclear reflection. U – undulator; HHLM – high-heat-load monochromator; CRL – compound refractive lens; SMS – the Synchrotron Mössbauer Source: HRM – high resolution monochromator, Def – Si (311) deflector, IB –the iron borate crystal inside the furnace with the four magnets and mounted on the Mössbauer transducer; KBM - Kirkpatrick-Baez mirrors; DAC – diamond anvil cell; D – avalanche photo diode detector. The SMS is optimized for highest intensity and best energy resolution, which is achieved by collimation of the incident synchrotron radiation beam and thus illumination of the high-quality iron borate crystal within a narrow angular range around an optimal position of the rocking curve. The SMS is permanently located in an optics hutch and is operational immediately after moving it into the incident beam. The bandwidth of radiation provided by the SMS is ~3 Γ 0 or ~15 neV (in real experiments we work with energy resolution lower than possible to have more intensity), the intensity is ~2×104 photons/s and the typical scanning velocity range is about ±12 mm/s (±0.6 µeV). In contrast to conventional radioactive sources, the SMS gives the possibility to focus the beam to tens of µm (the best achieved by now is 9x6 4.!Synopsis!(Scope!of!the!Thesis)! ! ! ! ! 38! µm2). As in-line monochromator SMS can be used with all existing sample environments in the experimental hutches downstream of the beamline. Figure 4.2.2 Spectra of (Mg0.857Fe0.2)O ferropericlase at a pressure of 29 GPa at room temperature (green circles) and at 1580 K (blue triangles). I implemented SMS in order to study electronic properties of materials under highpressure extreme conditions. One of the biggest SMS advantages is that it allows collecting spectra rapidly, on the scale of several minutes. For example, a high-quality Mössbauer spectrum of the silicate perovskite sample in the DAC at 93 GPa was acquired for about 10 min, compared to more than one week using a radioactive point source for the same sample. This, in combination with possibility to focus beam produced by SMS, enables to perform high-temperature studies using laser heating (Figure 4.2.2). The almost 100% recoilless resonant radiation delivered by the source and its high brightness allows a broad field of SMS applications. Due to a very strong suppression of electronic scattering for pure nuclear reflections (~10-9) SMS operation does not required any gating of the prompt electronic scattering. Thus, SMS can be utilized in any mode of storage ring operation. 4.!Synopsis!(Scope!of!the!Thesis)! ! ! ! ! 39! In this work SMS was used to study of the spin transition in Fe3+ ions in iron containing silicate perovskite, which is the dominant phase of the lower mantle of the Earth (by most recent reports ~93% (Murakami et al.)). 4.3 Spin state of Fe3+ ions in the lower mantle Using developed energy domain synchrotron Mossbauer source we conducted a study of several iron containing silicate perovskite samples (both Al containing and Al-free). We have studied four different silicate perovskite samples: Mg0.63Fe0.37Si0.63Al0.37O3 (~80% Fe3+/ΣFe); Mg0.63Fe0.37Si0.63Al0.37O3 (~70% Fe3+/ΣFe); Mg0.78Fe0.2Al0.05Si0.97O3+δ (~50% Fe3+/ΣFe); Mg0.94Fe0.06SiO3+δ (~20% Fe3+/ΣFe). In our experiments SMS spectra at room temperature were collected up to 122 GPa using diamond anvil cells, with or without laser annealing of the samples. Details of sample synthesis and experiment methodology are given Section 5.4. First, energy-domain Mössbauer study of Mg0.63Fe0.37Si0.63Al0.37O3 perovskite was performed. It contains iron dominantly as Fe3+, measured spectra are extremely well resolved (Fig. 4.3.1). We fit the data using model that consist of three quadrupole doublets, one assigned to Fe3+ and two assigned to Fe2+ (high and low quadrupole splitting, QS). Unlike in case of NFS studies, SMS allows unambiguously separate contributions for Fe3+ and Fe2+ based on doublets centre shifts (CS). As it is clear even from visual inspection of Mössbauer spectra (Fig. 4.3.1) there are no changes in the shape and relative area of the doublet corresponding to the Fe3+ over the entire pressure range. The hyperfine parameters of the low QS Fe2+ doublet (blue doublet in Fig. 4.3.1) corresponds to the high-spin state (McCammon et al., 2008), while the doublet with high quadrupole splitting (black doublet in Fig. 4.3.1) corresponds either to intermediate-spin (IS) Fe2+(McCammon et al., 2008) or a distortion of the site occupied by high-spin Fe2+ (Hsu et al., 2010). Irrespective of the interpretation of the Fe2+ spin state, our conclusions regarding the absence of a spin transition in Fe3+ remain the same. The fitting model obtained for sample #1 spectra was implemented for all other samples. The hyperfine parameters for Fe2+ and Fe3+ for all samples studied are 4.!Synopsis!(Scope!of!the!Thesis)! ! ! ! ! 40! the same within experimental error. Moreover there is no change in Fe3+/ΣFe for individual samples over the entire pressure range of the experiment (Fig. 4.3.2). The hyperfine parameters of the Fe3+ doublet are consistent with the high-spin state (Greenwood and Gibbs, 1971), and their smooth variation with pressure indicates that Fe3+ does not undergo any spin transitions within the entire pressure range. Notably the QS value reported for low-spin Fe3+ from both experimental (Catalli et al., 2010; Catalli et al., 2011) and theoretical (Hsu et al., 2011) studies (red dashed line, Fig. 4.3.2) are more than twice our observed values. ! Fig. 4.3.1 SMS spectra of Mg0.63Fe0.37Si0.63Al0.37O3 perovskite sample #1 at room temperature showing their evolution with pressure (a) 2.5 GPa; (b) 38 GPa; (c) 67 GPa; (d) 93 GPa. The data were fit to one Fe3+ doublet (red) and two Fe2+ doublets (blue and black), and the fit residual is shown below each 4.!Synopsis!(Scope!of!the!Thesis)! ! ! ! ! 41! spectrum. Area asymmetry is due to preferred orientation of the sample, and the velocity scale is given relative to α-iron. Figure 4.3.2 Pressure variation of FeAlPv hyperfine parameters (centre shift [CS] and quadrupole splitting [QS]) (a) high-spin Fe3+; (b) high QS Fe2+. Sample data are indicated as follows: Mg0.63Fe0.37Si0.63Al0.37O3 perovskite sample #1 (red and blue circles); Mg0.63Fe0.37Si0.63Al0.37O3 perovskite sample #2 (orange and blue squares); Mg0.78Fe0.2Al0.05Si0.97O3+δ perovskite (brown and dark blue circles); Mg0.94Fe0.06SiO3+δ perovskite (purple and green circles). The expected quadrupole splitting for low-spin Fe3+ is shown as a horizontal red dashed line. (c) Pressure variation of Fe3+/ΣFe as determined from the relative areas: perovskite sample #1 (red circles); Mg0.63Fe0.37Si0.63Al0.37O3 perovskite sample #2 (orange squares); Mg0.63Fe0.37Si0.63Al0.37O3 perovskite (brown circles); Mg 0.94Fe0.06SiO3+δ perovskite (purple circles). Horizontal lines are guides for the eye. Values obtained after laser heating of Mg0.63Fe0.37Si0.63Al0.37O3 perovskite sample #2 and Mg0.78Fe0.2Al0.05Si0.97O3+δ perovskite are indicated by grey stars. In all cases, parameters remained unchanged from those before heating within experimental error, demonstrating that there is no high-spin to low-spin transition in Fe3+ at the pressure-temperature conditions of the lower mantle Several studies (Catalli et al., 2010; Catalli et al., 2011; Fujino et al., 2012) have suggested that the reduced volume of LS Fe3+B could lead to a redistribution of Fe3+ from the Ato the B-site in the perovskite structure within the lower mantle. To 5.1!Multispace quantum interference in 57Fe synchrotron Mössbauer source! ! ! ! 48! crystal, the nuclear polariton generates a -ray beam. The distribution of the emitted quanta in time and in space is dictated by the temporal and spatial development of the nuclear polariton, which can exist inside the target in forms of either running or standing wave. The running wave mode is realized in the processes of the forward scattering, while the standing wave pattern is formed for nuclear Bragg reflections. The intensity of radiation emitted after each SR pulse is decaying in time. In space, the radiation forms usually the low-divergent beams both in the forward scattering and in Bragg reflection directions (Kagan, et al., 1979). The delayed resonant radiation is, however, preceded by a high-intense xray pulse contributing a huge non-resonant background in direction of the primary beam. Even after extreme monochromatization of the incident radiation (e.g., down to eV) the signal-to-noise ratio is still 10 - 10 ( with the natural width of nuclear resonance 5 neV). As for Bragg reflection, an x-ray photon can be reflected by a crystal through either nuclear resonant or Rayleigh electronic scattering processes. It is the electronic scattering that brings the large background in the Bragg direction on top of the nuclear resonant signal. Fortunately the nature of nuclear Bragg reflections offers a way out of this disappointing situation. The solution of the problem can be found using electronically forbidden but nuclear allowed Bragg reflections, which exist owing to an unusual polarization dependence of nuclear resonance scattering in the presence of hyperfine interaction. Such pure nuclear reflections were predicted for antiferromagnetic crystals (Tramell, 1961; Belyakov and Ayvazian, 1968) as well as for the crystals with electric field ordering (Belyakov and Ayvazian, 1969), and observed for the first time in refs. (Smirnov et al., 1960; Mirzababaev et al., 1971). It was a pure nuclear reflection that was applied in the first experiment where nuclear resonance excitation by synchrotron radiation was unequivocally observed (Gerdau et al., 1985). Since the pure nuclear reflectivity is intimately related to magnetic or electric hyperfine interaction, the nuclear array in a crystal behaves as a multiline radiator due to hyperfine splitting of nuclear levels. This is, of course, an inconvenient characteristic of a Mössbauer source, which can not be accepted. Luckily, a particular case of hyperfine interaction, which is well matched to the idea of the generation of a single-line Mössbauer radiation, does exist. Pure nuclear reflectivity within an energy 5.1!Multispace quantum interference in 57Fe synchrotron Mössbauer source! ! ! ! 49! band of about natural linewidth of the Fe nuclear excited level has been obtained when an iron borate FeBO single crystal was heated close to Néel temperature (Smirnov et al., 1986). Temporal properties of pure nuclear diffraction in the vicinity of were studied in Ref. Chumakov et al., (1990) using the pulsed structure of SR. Approaching the transition to the paramagnetic state at Néel temperature (for FeBO T is 348 K), the hyperfine structure of the Mössbauer diffraction spectrum for FeBO collapses and the reflection normally disappears above T due to destructive spatial interference. However, an application of an external weak magnetic field to the crystal results in a drastic transformation of the interference conditions. According to the quantum mechanical principle of superposition of states, after absorption of a SR x-ray photon every nucleus of the nuclear ensemble can be found on one of the four allowed sublelevs of the excited state. Furthermore, due to mixing the nuclear spin states, each of these four sublevels is characterized by two allowed spin projections. Thus, the emission of a delayed gamma-ray photon is described by the multi-space interference of the relevant probability waves, where geometrical, energy and spin domains are intrinsically involved. The combined multipath interference in space, energy, and spin domains results in a pseudo singleline resonance structure, which provides a basis for the creation of a single-line synchrotron Mössbauer (SM) source. An analysis of the hyperfine structure under these conditions is given in Ref. Smirnov et al. (2000). The first source of SR-based Mössbauer radiation has been developed (Smirnov et al., 1997) in 1997 at the European Synchrotron Radiation Facility. The source emits a directed beam of fully recoilless and polarized radiation. These properties have been effectively used in the first application of the SM source for studying magnetic field distribution in iron alloys (Pankhurst et al., 2001). A large progress in the further development of the SM source has recently been achieved at SPring-8 (Mitsui et al., 2009). The physics of SMS is a fascinating example of multispace quantum interference, i.e. the interference of a gamma-quantum in geometrical, energy, and spin spaces. This paper is devoted to the detailed analysis of this coherent processes, which determine the properties of the SM radiation. Namely, the angular, energy, and time distributions of the SM radiation are analyzed in order to define necessary 5.1!Multispace quantum interference in 57Fe synchrotron Mössbauer source! ! ! ! 50! conditions for an optimal performance of the SM source. The remainder of the paper is organized as follows. Section 1 gives a detailed analysis of the process of the multispace interference of nuclear resonant scattering of SR by the unit cell of 57FeBO3. In section 2, properties of the nuclear polariton for the diffraction process of radiation by the nuclear array are considered. In Section 3, the angular, energy, and time distributions of the Fe SM radiation are calculated. A summary and conclusions are given in Section 4. 5.1.3 Multispace interference of γ-ray photon in the unit cell of 57FeBO3 crystal Table 5.1.1 Nuclear transitions between the ground and excited states with magnetic quantum numbers for various transition energies and various nuclei in the unit cell of the IB crystal. . We consider the emission of a -ray photon by the two Fe nuclei belonging to the unit cell of FeBO - Iron Borate (IB) - crystal. Let the nuclei be excited by a short pulse of synchrotron radiation much shorter than the lifetime of the excited nucleus. Then the processes of nuclear excitation and de-excitation are two sequential, temporally well decoupled, events. After the prompt absorption of a primary photon and some dwelling in the intermediate excited state, the transition back to the ground 5.1!Multispace quantum interference in 57Fe synchrotron Mössbauer source! ! ! ! 51! state occurs with emission of a secondary photon. In accordance with the general principles of excitation, both nuclei are excited in the intermediate state with equal probability amplitudes, the phases of which in the case of a plane synchrotron radiation wave are determined by the scalar product , where is the wave vector of the incident radiation and is the vector indicating the position of a nucleus in the unit cell ( =1,2). In order to calculate the emission in the direction , one should add up the amplitudes of the two -ray wavelets with the account of their relative phase, which is determined by the difference in their corresponding optical paths. The phase difference is then given by the expression . Thus, in order to evaluate the result of the interference in geometrical space, one has to consider two paths. The phase difference of waves scattered along the two paths plays a crucial role for the intensity of emission. Each of the two spatial paths is branched further into separate paths within the energy domain. Due to magnetic dipole interaction of the nuclear spin with the internal field, the nuclear levels in the IB crystal are split into sublevels. The level of the ground state having nuclear spin splits into two sublevels; and the level of the excited state having spin splits into four sublevels. The selection rules for magnetic dipole transition (M1), relevant to our case, allow one to observe six nuclear transitions between the ground and excited states with the change of the magnetic quantum number . Here are the magnetic quantum numbers for the excited and ground nuclear states, respectively. The IB crystal is a canted antiferromagnet. The magnetic and crystalline unit cells of the crystal are the same (like for Fe2O3 hematite crystal, see Fig. 16 in Ref. Shull et al. (1951). The magnetic fields acting upon nuclei in the unit cell are equal in magnitude but almost opposite in direction. For this reason, the sublevels of the excited and the ground states have the same energies for both nuclei but the states appropriate to the equivalent sublevels are characterized by the opposite signs of the magnetic quantum numbers. Therefore, the transitions at the st and nd nuclei with the same resonant transition energies ( ) have the opposite signs in the change 5.1!Multispace quantum interference in 57Fe synchrotron Mössbauer source! ! ! ! 52! of the spin magnetic quantum numbers , see Table 5.1.1. Here the spin quantization axis is chosen to be directed along the magnetic field at the st nucleus. For each arbitrary energy of the radiation spectral component all six transitions will give its specific contribution to the interference field depending on the energy distance between and a resonant energy , where . Here one definitely faces a bright case of inter-resonance nuclear interference. For the Lorentzian shape of each resonance, the summary amplitude of scattering is then proportional to , where stands for the energy deviation from th resonance in the units of the resonance natural half-width (for the sake of simplicity we have assumed so far that the amplitudes of different transitions are equal to each other). Thus, for the two nuclei in the unit cell the combined process of -ray interference in geometrical and energy spaces involves twelve contributions. The arrangement of atoms in the IB crystal does not only result in the formation of the internal magnetic field at the iron sites but also in the creation of a non-uniform crystalline electric field. The electric field gradient (EFG) in the crystal exhibits axial symmetry. The principal axis of the EFG tensor is orthogonal to the internal magnetic field at both iron sites. The Fe nucleus in the first excited state possesses both a magnetic and a quadrupole moment, while in the ground state it has only a magnetic moment. So, the nucleus experiences a pure magnetic dipole interaction in the ground state and a combined magnetic dipole and axially symmetric electric quadrupole interaction in the excited state. 5.1!Multispace quantum interference in 57Fe synchrotron Mössbauer source! ! ! ! 53! Figure 5.1.1 Amplitudes of contributions of the pure spin states , into the mixed spin states for different energy sublevels of the excited nuclear state of different nuclei in the unit cell of the IB crystal, see Table II. For parameter see explanation to Eq. (5.1.1). The splitting of nuclear states in an IB crystal under conditions of the combined magnetic dipole and electric quadrupole hyperfine interaction has been analyzed in ref. Smirnov (2000). The four dimensionless energies of the hyperfine interaction in the excited state are given by (5.1.1) where , is the energy of interaction of the magnetic moment of the nucleus in the excited state with the internal magnetic field ( is the nuclear -factor in the excited state, is the nuclear magneton) and is the energy of interaction of the nuclear electric quadrupole moment with the electric field gradient ( is the electric field potential at the nucleus, is the charge of proton); are given in the units of the quadrupole interaction energy . Since in the excited state of Fe the -factor is 5.1!Multispace quantum interference in 57Fe synchrotron Mössbauer source! ! ! ! 54! negative, the sub-level energies are ordered in the following way: . Concerning the value of the internal magnetic field, it is a function not only of temperature but also of the applied magnetic field. The application of the external field plays a significant role in the near vicinity of Néel temperature where antiferromagnetic ordering of the atomic spins can be induced in the crystal (BorovikRomanov and Ozhogin, et al., 1960). For pure magnetic interaction pure spin states (i.e., the states with a defined spin projection ) can exist. But under conditions of combined magnetic and electric hyperfine interaction the excited nuclear states may be mixed over the spin projections. In the particular case of the axially symmetric EFG with the main axis orthogonal to the magnetic field and z-axis directed along the magnetic field, each nuclear excited state is mixed over two spin projections. In this case the Hamiltonian eigenvalues given by Eq. (5.1.1) are appropriate for the eigen functions of the excited states represented in Table 5.1.2. Table 5.1.2: The mixed-spin eigen functions for various excited nuclear states with the eigen energies and for various nuclei in the unit cell of the IB crystal. 5.1!Multispace quantum interference in 57Fe synchrotron Mössbauer source! ! ! ! 55! The excited nuclear states with energies are mixed over spin projections , whereas the states with energies are mixed over spin projections Table 5.1.2 shows that only four amplitudes are required to describe the eigen functions of all excited states for both nuclei. Following the analysis presented in Ref. (Smirnov, 2000), these amplitudes can be obtained in an explicit form (5.1.2) They are defined by a single parameter . The magnetic hyperfine field in the IB crystal is a function of temperature. Approaching the transition to the paramagnetic state at Néel temperature , the field magnitude rapidly decreases. At room temperature, the magnetic interaction dominates over the electric one, parameter , and the excited nuclear states are nearly pure spin states with and . On the contrary, in the range of the excited states are already significantly mixed over spin projections. The evolution of the mixed-spin states is illustrated in Fig. 5.1.1. The dependences of the pure spin state contributions into the mixed states are shown within the range of where the magnetic dipole and electric quadrupole hyperfine interactions are getting comparable. Figure 5.1.1 clearly shows that in approaching Néel temperature ( ), the excited states are getting strongly mixed over spin projections. Because the two spin projections are mixed in each excited substate the resonance scattering via a separate nuclear transition in every nucleus branches out further into the four paths. 5.1!Multispace quantum interference in 57Fe synchrotron Mössbauer source! ! ! ! 56! Figure 5.1.2 Scattering paths via the pure spin states of the excited nuclear sub-state having energy in the 1st nucleus of the IB crystal unit cell. On the upper panel the transitions, up and down, between the ground and the excited states are displayed. On the lower panel four possible paths are shown, where changes of spin projections in different paths are indicated. are the changes of magnetic quantum number in separate transitions; are the magnetic quantum numbers for the excited and ground nuclear states respectively. For elastic scattering, the process considered here, the initial and final states are the same. Therefore, if measurements do not permit to establish, which intermediate spin state was involved in the scattering, the scattering paths related to all intermediate states should interfere. Here one meets an interesting case of intraresonance nuclear interference in the spin domain. Fig. 5.1.2 shows an example of the scattering paths via the nuclear transition with the lowest transition energy for the -st nucleus. As it is seen from the figure, in order to build up the interference field for a given transition in a single nucleus, four wavelets of the same frequency but different in polarization states and scattering amplitudes should be added. The amplitude of a wavelet for -th nuclear transition in -th nucleus contains the complex G - factor given by the following general expression (5.1.3) 5.1!Multispace quantum interference in 57Fe synchrotron Mössbauer source! ! ! ! 57! where designates a Clebsch--Gordan coefficient, which is a probability factor for the transition between sublevels of the ground and excited states taking into account the angular momentum transfer for the (nucleus+photon) system; it also depends on the spins of the ground and the excited states and on their zprojections. In this manuscript we are considering diffraction with (M1) multipolarity (magnetic dipole transition) and have chosen the magnetic basis for the polarization vector: is the scalar product of the magnetic polarization vector of the wave propagating in the direction ( stands for the direction along the incident wave, - for the direction along the exit wave), with polarization (vectors lie in the scattering plane, while vectors are perpendicular to it) and the spherical unit vector in the coordinate system related to the hyperfine fields at th nucleus in the unit cell: , where are mutually orthogonal unit vectors related to the hyperfine fields in the unit cell, is directed along vector (see Fig. 5.1.3); finally, are the amplitudes of spin states in the th sub-level of the excited nuclear state. Below the - amplitudes are related to the defined above amplitudes. (5.1.4) In the example, depicted in Fig. 5.1.2, the interference in spin space leads to the following expression of the -factor (5.1.5) 5.1!Multispace quantum interference in 57Fe synchrotron Mössbauer source! ! ! ! 64! resonance interference starts playing the essential role even at room temperature. On the overlapping slopes of the 1st and the 3rd lines, as well as of the 4th and the 6th lines the deep and sharp valleys (i.e., minima of the scattered intensity) are observed which are due to the resonance absorption of radiation at the energies . For the transitions M = 0 the nuclear diffraction is cancelled due to destructive interference of the constituting waves in geometric space. When due to the crystal heating the internal field decreases down to kOe, the role of the inter-resonance interference rises significantly, Fig. 5.1.4b. The effect of the interference is seen in a much broader angular range. The interference causes a strong asymmetry in both energy and angular distribution of the emitted radiation. Beyond the Bragg angle range the energy spectrum contains only three peaks at the lower angles, - the 3rd line is suppressed over there, while at the higher angles the four lines are present but with the strongly weakened inner ones. Within the Bragg angle range a complicated interference pattern is observed where one can still distinguish the four lines of about equal strength. On the energy scale, off the resonance range far extended wings attract attention. They are also present on the former figure. The wings represent an axial symmetric configuration. Such a symmetry is due to varying conditions of the interference of the waves scattered by nuclei and is related to the fact that the real part of the nuclear amplitude is uneven function of the energy deviation from resonance. Finally, we come near to the Néel point with the magnetic field of only 2 kOe (Fig. 5.1.4c). The multi-line spectrum of the emitted radiation has significantly collapsed here. In the vicinity of the Bragg angle a pseudo single line resonance structure is observed, which is created by the destructive inter-nuclear interference (in geometrical space) as well as by a strong inter-resonance (in energy domain) and intra-resonance (in spin-domain) interference of radiation components discussed in the previous section. As seen in Fig. 5.1.4 emission occurs over a range of settings of the crystal around the Bragg angle. To reveal the details of the interference pattern in the following section we have analyzed the angular function of the coherent emission and determined the angular breath of strong emission for different magnitudes of the internal magnetic field. In addition the spectral composition and the time distribution of -ray photons radiated by the nuclear array in SM source were examined. 5.1!Multispace quantum interference in 57Fe synchrotron Mössbauer source! ! ! ! 65! 5.1.5 Angular, energy and time distributions of Fe SM radiation Initially we discuss the geometrical properties of the interference pattern. First of all we wish to know how the intensity of the radiation emitted by the crystal depends on its angular setting in the vicinity of the Bragg angle. To estimate the theoretical angular width of the emission function for a perfect crystal we can neglect the angular width of the incident beam, assuming a plane wave is incident on the crystal. To find in this way the emission angular function we have to integrate the two dimensional emission function (for its definition see text after Eq. (5.1.14)) over frequency, assuming SR intensity to be frequency independent value over the whole range of the nuclear resonance. The emission angular function is then given by the following integral (5.1.15) where in the intensity function the angular and frequency variables are replaced adequately: and . One should underline that the expression of Eq. (5.1.15) gives the emission angular dependence integrated over the whole resonance range. The emission angular functions for different magnitudes of the internal magnetic field are displayed in Fig. 5.1.5. The curve on the bottom panel of Fig. 5.1.5 having a symmetric shape represents the angular dependence of emission at room temperature. The center of angular range of pronounced emission is shifted with respect to the angle given by the original Bragg reflection law by 15.5 rad. This shift is due to refraction of the incident beam at the entrance into the crystal. The width of the reflectivity curve is very small, 5 rad. When heating the crystal towards Néel temperature the breadth and shape of the emission angular function is drastically changed. Three main features can be recognized (Smirnov, 2009): • significant decrease of the emitted intensity, • strong broadening of the angular range of emission, 5.1!Multispace quantum interference in 57Fe synchrotron Mössbauer source! ! ! ! 66! • creation of the double hump structure of the angular curves with the dip exactly at the Bragg angle corrected for refraction. Figure 5.1.5 The emission angular dependence of radiation from the IB crystal integrated over the whole resonance range. The angular setting of the IB crystal is changed in the vicinity of Bragg angle for pure nuclear reflection (333). The angular functions are displayed for different values of the internal magnetic field decreasing from the bottom to the top in approaching Néel temperature. At the lowest field (i.e., the highest temperature of the crystal) the emission intensity is only of the order of of the intensity at room temperature. Such diminution of the coherent emission near Néel temperature was expected because of the destructive nature of interference at this temperature. Along with the decrease of the coherent emission the angular range of emission is being significantly enlarged: from rad at the field of kOe (the bottom panel in Fig. 5.1.5) up to rad at the field of kOe (the upper panel in Fig. 5.1.5). This happens because the strong off-resonance wings of nuclear scattering (located mainly in the narrow angular interval around the exact Bragg position, i.e., around the peak of the rocking curve, see Figs. 5.1.4a,b) become heavily suppressed by destructive interference when temperature approaches (Fig. 5.1.4c). On the 5.1!Multispace quantum interference in 57Fe synchrotron Mössbauer source! ! ! ! 67! contrary, the wider angular distribution of the on-resonance scattering is less affected by destructive interference (Fig. 5.1.4c). Thus, it provides the wider rocking curve around . It is of interest to note that a further decrease of the field and of the related emission intensity is not accompanied any longer by a change of the shape and the width of the angular curve. A kind of saturation is reached in the breadth of the angular range of emission of rays over the entire resonance region near the Néel point. Fig. 5.1.4c shows that the outer wings of the landscape falls abruptly in the narrow angular interval near the Bragg angle. Computer analysis of the emission function shows that the wings are shifted out of the immediate Bragg range more and more as the collapse of the hyperfine structure occurs. They start therefore to give their contributions to the emitted radiation only when the angle of incidence is shifted from the central zone to the sides. This is the reason for the formation of the double hump structure and the appearance of the dip in the emission angular function. The angular distribution of the emitted radiation is determined both by the emission angular function and by the instrumental function, i.e., by the angular distribution of synchrotron radiation in the incident beam. When the angular divergence of the exciting radiation is large in comparison with the angular width of the range of emission, the emission occurs throughout the whole allowed range. If the SR intensity is a constant value in this range, the angular distributions of the emitted radiation just repeat those depicted in Fig. 5.1.5. In the opposite case of very small divergence of the exciting radiation in comparison with the angular width of emission, the angular distribution of radiation reproduces the instrumentation function form. To get the angular distribution of the emitted radiation in an intermediate case one must find the product of the emission angular function and of the instrumental function at an arbitrary angle of incidence of the exciting synchrotron radiation. An example of the angular distribution of the emitted radiation is shown in Fig. 5.1.6, assuming the instrumental function to be of Gaussian form . The nuclei are excited by SR exactly at the dip of the emission angular function, rad and rad. The angular 5.1!Multispace quantum interference in 57Fe synchrotron Mössbauer source! ! ! ! 68! distribution of radiation emitted in the vicinity of Néel temperature for the internal magnetic field of 2 kOe is compared with the instrumental angular function. As seen in the figure, under the considered real conditions, the angular breadth of the emitted radiation exceeds slightly the angular width of the instrumental function. The top of the distribution is slightly modulated by the emission angular function. We turn now to the energy distribution of the emitted radiation for a fixed angle of incidence of the exciting SR beam. At the beginning we consider a simple model of the destructive inter-resonance interference. As shown in ref. Smirnov (2000), due to the combined multispace interference only two resonance lines survive close to Néel temperature. They belong to the 3rd and 6th resonances in the original hyperfine structure in the FeBO . The energies of the relevant transitions nearly coincide and the strengths of the lines are almost equal. Thus the spectrum in our model can be presented by the sum of two Lorentzians, each renders a single line radiation emitted at a separate resonance transition, (5.1.16) As , what corresponds to a collapse of the hyperfine structure, the emission intensity (as the integrand in Eq. (5.1.15)) vanishes. Under these conditions the double peak structure is being transformed into a pseudo-single line, which shape is given approximately by the following expression Figure 5.1.6 Angular distribution of the emitted radiation - solid line; angular distribution of the exciting synchrotron radiation -dots (see text). The first distribution is scaled in intensity for comparison with the second one. The crystal is set at the position of angular dip (top panel in Fig. 5.). (6.1.17) 5.1!Multispace quantum interference in 57Fe synchrotron Mössbauer source! ! ! ! 69! where . Thus the energy distribution is presented in our model by a Lorentzian-squared function. The line width in this case is less than , namely . One should note that the interference is constructive at the energies lying in between the resonance ones and , while, in contrast, it is destructive on both left and right outer sides of resonance region (the amplitudes in Eq. (5.1.16) tend to cancel each other over there). Whence, -ray emission completely disappears when (above Néel temperature). However, in the real case the application of a small external magnetic field restores the antiferromagnet structure of the crystal and -ray coherent emission resumes (Chumakov et al., 1986). Due to coherent effects the real width of the energy distribution can be larger than . To obtain the energy distribution of the emitted radiation at the angle of incidence of one should calculate the following integral (5.1.18) All the values entering into Eq. (5.1.18) are defined above. Several energy distributions of radiation emitted near Néel temperature at different angles of incidence of the SR are displayed in Fig. 5.1.7. The central part of the emission angular function around rad is shown on the upper panel in Fig. 5.1.7, where the considered angular interval is limited by the L,R bars. It is mostly the region in between the two humps of the angular curve. The divergence of the exciting SR beam is taken as 5 rad. 5.1!Multispace quantum interference in 57Fe synchrotron Mössbauer source! ! ! ! 70! Figure 5.1.7 Energy distributions of the emitted radiation at different angles of incidence of the exciting SR in the vicinity of the dip zone of the emission angular function for the (333) reflection and for the internal magnetic field of 2 kOe. The dip zone (taken from Fig. 5.1.5) is shown on the upper panel. The energy distributions on the lower panels refer to the left and the right sides of the dip zone, L and R respectively, as shown on the upper panel. The energy distribution at the exact dip position is displayed on both L,R panels - bold line curves. Each next distribution corresponds to an angular shift of the crystal from the dip position by 5 rad. For better visualization the energy spectra are equally spaced along the vertical axis. The step in change of the angle of incidence is of the same value. The energy distributions of the emitted radiation are shown on the lower panels. The distributions corresponding to the shifts towards the lower hump of the emission angular function (to the left of the dip zone) are denoted by and those towards the higher hump (to the right of the dip zone) are denoted by . The energy distribution of the emitted rays in the case of excitation of the nuclear array at the exact dip position is represented by the bold-line curve on both and graphs. The nature of the double hump structure in the emission angular function prompts the idea to select just the dip zone for obtaining a single line SM radiation spectrum. Indeed, as seen from the figure, in this range it is predominantly a single line having a width close to (horizontal bar on the half-height). A strongly suppressed satellite is noticed on the left-hand side of the main peak. When moving off the center of the angular curve to its left hump a broader line is being formed in the range of the lower energies of the radiation spectrum. Its relative contribution is rising as the angular shift is increasing. When moving off the center of the angular 5.1!Multispace quantum interference in 57Fe synchrotron Mössbauer source! ! ! ! 71! curve towards its right hump the changes of the radiation spectrum are mostly the same but now a broader line appears at the right-hand side of the central maximum and the small satellite is not smoothing out as in the L distributions. Appearance of the side broad lines in both L and R spectra is due to excitation of the nuclear array at the angles of incidence at which the wings the emission landscape in Fig. 5.1.4c are already touched. Figure 5.1.8 Time distributions of the emitted radiation at different angles of incidence of the exciting SR in the vicinity of the dip zone of the emission angular function for the (333) reflection and for the internal magnetic field of 2 kOe. The dip zone (taken from Fig. 5.1.5) is shown on the upper panel. The time distributions on the lower panels refer to the left and the right sides of the dip zone, L and R respectively, as shown on the upper panel. The time distribution at the exact dip position is displayed on both L,R panels - bold line curves. Each next distribution corresponds to an angular shift of the crystal from the dip position by 5 rad. For better visualization the time distributions are equally spaced along the vertical axis. Finally we come to the angular resolved time distributions of the emitted radiation. They were calculated by performing the following integration (5.1.19) where t is real time and is the natural lifetime of the excited nuclear state. The inner integral represents the inverse Fourier transform of the energy distributions of the emitted radiation amplitudes. The obtained time distributions are displayed in Fig. 5.1.8. This figure is delineated in the same way as the former one. All time dependences overthere correspond to the energy distributions shown in Fig. 5.1.7. The time dependence of the emitted radiation in the case of excitation of the nuclear 5.1!Multispace quantum interference in 57Fe synchrotron Mössbauer source! ! ! ! 72! array at the exact dip is marked by the bold lines on both and graphs. The nonexponential shape of this dependence reveals the true nature of a single line spectrum obtained nearby the Néel point. In fact it is a pseudo-single line formed in the process of the combined interference of radiation paths in the geometric space, and in energy and spin domains. In particular, the -ray waves emitted by nuclei in the unit cell right upon excitation cancel each other, because these waves are of equal amplitudes and opposite in sign due to the phase shift between them of at the zero time. The relative phase between the waves is changing due to a difference in their carrier frequencies with time towards their constructive interference, therefore a quantum beat pattern could be observed. However, at the same time the coherent decay of the excited state is proceeding, so that the quantum beat and decay processes are overlapped in time. In our model of interference of the two Lorentzian lines, see Eq. (5.1.16), using the inverse Fourier transform, we can obtain the following time dependence (5.1.20) where quantum beating and decay are presented by the pre-exponential factor and the exponential one, respectively. Quantum beat period here is . The resultant pattern depends strongly on the relation between the characteristic times of the two processes, the beat period and the natural decay time . In the case where the beat period and decay time are of the same order the two processes compete. In the real case one should consider the relation between the beat period and the characteristic time of the coherent emission . In the vicinity of Néel temperature the beat period can be several times larger than , since in this temperature range the hyperfine splitting is already fractional of breadth of the coherent resonance line. The transition from destructive to constructive interference proceeds here slower than the decay. In this approximation the following time dependence is valid: . The form of the time dependences in Fig. 5.1.8 is still determined by the competing processes. Increase of the intensity due to positive-going inter-resonance interference is compensated by the speeded up coherent emission. This competition results in the formation of the emission maximum in the time dependences. When approaching the times of fully constructive 5.1!Multispace quantum interference in 57Fe synchrotron Mössbauer source! ! ! ! 73! interference most of the stored energy is released already through the incoherent channels (Chumakov et al, 1990). When moving from the central angular zone towards the humps the time distributions are determined in addition to the discussed reasons by the interference of the narrow central component with the broader lines at the sides in L and R graphs of Fig. 5.1.7. 5.1.6 Summary and conclusion Electronically forbidden but nuclear-allowed Bragg reflections can be employed to generate the recoilless radiation by exciting nuclear array to its isomer level with the help of the synchrotron radiation. Pure nuclear reflections exist owing to a strong spin dependence of nuclear scattering in the presence of hyperfine fields in crystals. This way a source of Mössbauer radiation alternative to the radioisotope sources can be realized. The coherent excitation of the nuclear array leads to the creation of nuclear exciton polariton states which generate at the exit of the crystal the coherent radiation free of a non-resonant contribution. A particular case of pure nuclear reflection is considered where a single line Fe Mössbauer radiation is generated. It is accomplished in an antiferromagnet 57FeBO3 crystal (Iron Borate (IB) crystal) heated up to the vicinity of its Néel temperature. When considering the interference of radiation while nuclei in the unit cell of the IB crystal de-excite one can find 48 contributions into the interference amplitude. Two contributions come from the de-excitation of the two nuclei present in the unit cell. Each of them contains six contributions in the energy domain where nuclear levels are split due to combined magnetic and electric hyperfine interaction in the IB crystal. Finally near Néel point where the magnetic dipole and electric quadrupole interactions in the IB crystal are of comparable strength the nuclear sublevels in the excited state become strongly mixed in spin projections. The two projections of nuclear spin are allowed at each sub-level. Therefore each of the 6 contributions in the energy domain involves in addition the 4 contributions in the spin domain. Thus the combined multi-path interference in geometric space, in energy, and spin domains 5.2!Angular, spectral and temporal properties of nuclear radiation 57Fe ! Synchrotron Mössbauer source.! ! ! ! 80! the crystal setting relative to the incident beam. The relevant angular dependence of the reflection intensity is referred as rocking curve. Figure 5.2.1 Schematic view of the experimental set-up. SR - beam after HRM; T, Mössbauer transducer which used for oscillation of the crystal; F - furnace combined with magnets arrangement,which contains 57FeBO3 crystal; H - magnetic field of 110 eO; A - standard single line absorber K2Mg57Fe(CN)6 ; D - APD detector. The beam reflected from the crystal was incident onto a single line absorber (A) and then to detector (D) (Fig. 5.2.1). To monitor the energy distribution of radiation emitted by the iron borate crystal we used a standard Mössbauer K2Mg57Fe(CN)6 single line absorber, which had an area density of resonant Fe isotope of 50 mg/cm and line width of 2.1 ( is the natural width of the resonance line). All measurements were performed for (333) PNR of the iron borate crystal. Because there is no difference in physics of nuclear scattering between different PNR in FeBO , the obtained results are applicable to any of them, with a correction on the difference in the rocking curve width for each reflection. The external magnetic field of 110 Oe was applied to the crystal in the direction normal to the scattering plane in order, first, to maximize the reflection coefficient, second, to broaden the magnetic transition region. We used the furnace and the magnets arrangement of a special construction (Potapkin et al., 2012) in order to avoid gradient of the temperature and of the magnetic field. Thanks to the very strong suppression of electronic scattering for PNR (10 ), it was possible to perform all measurements without any gating of the prompt electronic scattering. For more details concerning the experimental setup or crystal environment see Ref. (Potapkin et al., 2012). 5.2!Angular, spectral and temporal properties of nuclear radiation 57Fe ! Synchrotron Mössbauer source.! ! ! ! 81! 5.2.4. Results and Disscusion Fig.2 shows the rocking curve measured at room temperature (a) and at 348.9 K (b), i.e., slightly above 348.35 K. At room temperature, the full width at halfmaximum (FWHM) of the angular depend ence (Fig. 5.2.2a) was 10.5(2) rad. The theory predicts FWHM of 5.9(2) rad at room temperature. When the FeBO crystal was heated approximately to its Néel temperature the rocking curve width grew by a factor of 3, from 10.5(2) to 32.1(2) rad. Furthermore, the rocking curve acquired double-peak structure (Fig. 5.2.2b). The angular dependence near becomes asymmetric; the peak at the smaller angles is less than that at the bigger angles. Figure 5.2.2 Angular dependence of the reflected radiation for iron borate (333) pure nuclear reflection of IB crystal (a) at RT (b) 348.9 K. Circles shows experimental points, solid line shows theoretical fit. On figure 2(b) letters (from a to d) mark angular position of the crystal relative to the incident beam in which energy and time distribution was measured. To compare the experimental results with the theoretical predictions, the experimental data were fit with the use of code based on the theoretical formalism (Smirnov et al., 2011). The two variable parameters were used in the fitting of rocking curves, namely, the internal magnetic field H and the parameter which includes divergence of the incident beam and possible bending of the crystal. In the experiment divergence of the incident beam was fixed. Thus, variation of could simulate only bending of the real crystal with respect to the perfect one. The best fit for the rocking curve at room temperature was obtained with = 5.3(2) rad (Hint was fixed to 330 kOe, which is the room temperature value of in 5.2!Angular, spectral and temporal properties of nuclear radiation 57Fe ! Synchrotron Mössbauer source.! ! ! ! 82! Hint the iron borate crystals). The increase of the effective divergence from 2 rad to 5.3 rad reveals the slight crystal bending. In vicinity of the Néel temperature the best fit for the rocking curve was reached with H =12 kOe and = 5.9(2) rad. The obtained value of internal magnetic field was used as fixed parameter for fitting the energy and the time distributions of the emitted radiation in the range of the Bragg angle near the Néel temperature. Figure 5.2.3 Energy distributions of nuclear Bragg diffraction for iron borate (333) reflection measured in different angular position of the crystal relative to the incident beam. Olive circles - experimental points, red line - theoretical fit. Energy distribution was measured using standard Mössbauer K2Mg57Fe(CN)6 single line absorber. Measurements performed at four angular positions (Fig. 5.2.2b): small peak (a), minimum (b), big peak (c) and the right slope of the big peak (d). The measurements of energy and time distributions were performed at a series of angular positions along the rocking curve. Here we present the measurements taken at the four angular positions indicated in (Fig. 5.2.2b): small peak (a), minimum (b), big peak (c) and the right slope of the big peak (d). Mössbauer spectra of the single line absorber A measured in the given angular positions are displayed in Fig. 5.2.3. With the account for the absorber line contribution the 5.2!Angular, spectral and temporal properties of nuclear radiation 57Fe ! Synchrotron Mössbauer source.! ! ! ! 83! obtained spectra characterise the energy distributions of gamma radiation emitted at the given angles. The energy distribution in minimum (b) exhibits only one line (Fig. 5.2.3b). For brevity we will call it the main line. The FWHM of this line is 2 (10 neV). This value was found by subtracting the width of the single line absorber from the measured FWHM. In the angular range of the lower peak of the rocking cureve (Fig. 5.2.2a position a), the main line is accompanied by a satellite line (Fig. 5.2.3a) with a smaller energy. The lines are separated by energy interval ( ) 9 . With increase of deviation from the Bragg angle the intensity of the low energy satellite increases. In the angular range of the higher peak of the rocking curve a satellite with the energy larger than the energy of main line appears, see, for example, the two energy distributions taken at the peak and at its right slope (Figs. 5.2.3c,d). The theory predicts that energy of this satellite should be very close to the energy of the main line. Due to the limited energy resolution in the experiment the satellite was not resolved in the measured energy distributions of radiation. Only the growing asymmetry of the main line (Fig. 5.2.3c) from the side of bigger energies indicates the presence of the satellite. The difference in energy between the main line and the satellite is 2.5 . The satellite height is rising as the deviation from the Bragg angle increases. In the angular position 'd' Fig. 5.2.2 the satellite even dominates the main line. The energy distribution of the emitted radiation in this angular range looks just like a single broad line (Fig. 5.2.3d). Fig. 5.2.4 shows the time distributions of the emitted radiation in the same angular positions of the crystal relative to incident beam. In the range of smaller angles, for example on the top of the lower peak (position 'a' Fig. 5.2.2), the time distribution (Fig. 5.2.4a) exhibits quantum beats (Rütter et al., 1990), where positions of the maxima are controlled by separation of the main line and the satellite in the energy spectrum (Fig. 5.2.3a). In this particular case the time distribution follows the dependence (Smirnov et al., 2011): (5.2.1) where, τ is an effective decay time, which is determined by the coherent gamma-ray emission. The period of quantum beats given by equation is 98 ns. 5.2!Angular, spectral and temporal properties of nuclear radiation 57Fe ! Synchrotron Mössbauer source.! ! ! ! 84! Therefore, for the two lines with = 9 the first bump should be located at 49 ns and second bump at 147 ns. This is approximately what the measured time distribution shows (Fig. 5.2.4a). Figure 5.3.4 Time distributions of nuclear Bragg diffraction for iron borate (333) reflection measured in different angular position of the crystal relative to the incident beam. Olive circles shows experimental points, red line - theoretical fit, blue dash line shows fit to data using Eq. (5.2.2). Measurements performed at four angular positions (Fig. 5.2.2b): small peak (a), minimum (b), big peak (c) and the right slope of the big peak (d). In this case, the quantum beats are dumped due to a shortened lifetime - , which is related to an enlarged width of the energy lines as . That's why beats attenuate faster. In the minimum of the rocking curve (position 'b' Fig. 5.2.2) the time distribution does not show an exponential decay (Fig. 5.2.4b), which is expected in case of a Lorentz-like line. This indicates that single line is not a simple Lorentzlike line. In the minimum, the time distribution can be approximated by the dependence (Smirnov et al., 2011): (5.2.2) In this case bump position is defined by value of , i.e., . The fit to data by Eq. 5.2.2 is plotted by a dash line on Fig. 5.2.2b. Thus, the time distribution in the 5.2!Angular, spectral and temporal properties of nuclear radiation 57Fe ! Synchrotron Mössbauer source.! ! ! ! 85! minimum of the rocking curve is an overdumped beats case, where quantum beats are overdumped by exponential decay. In the range of bigger angles, for example in big peak position or right slope (positions 'c' and 'd' on Fig. 5.2.2b) the time distribution shows an intermediate case (Figs. 5.2.4c,d) between overdumped beats case and quantum beats case. There time dependences already do not follow Eq. (5.2.2). In this case, position of the bump is still possibly mainly determined by . One variable parameter was used in the fitting of energy and time distributions of reflected radiation, namely, the angle of incidence of the exciting synchrotron X radiation. The good fit to all the energy and time distributions was obtained, see Fig. 5.2.3 and Fig. 5.2.4. This indicates a good agreement between the experimental results and the theoretical prediction. The time dependence would follow the Eq. 5.2.1 if the main line is created in the process of destructive interference between two emission lines, in case where energy difference between two lines, i.e, converge to 0. In such case resulting line doesn't follow Lorentzian distribution. In first approximation, main line can be described by squared Lorentzian distribution(Smirnov et al., 2011): (5.2.3) where is the width of each of the interfering lines and is a value of resonance energy. However, if the main line would exactly follow the squared Lorentzian distribution, then for main line with width , i.e., for 70 ns, peak ( ) in time spectra should be at 140 ns and not at 25 ns like we have at measured time spectrum (Fig. 5.2.4b). Because, it turns out that in reality the main line is following more complicated dependence. The shift of the peak to lower times indicates that tails of main line dependence drop slower than for squared Lorentzian distribution. 5.2.5. Summary We have studied properties of gamma radiation emitted by nuclei in iron borate crystal set in the vicinity of Bragg angle for it pure nuclear reflection. It is found that the emission intensity measured at different angular positions of the crystal 5.2!Angular, spectral and temporal properties of nuclear radiation 57Fe ! Synchrotron Mössbauer source.! ! ! ! 86! relative to the exciting synchrotron radiation beam exhibits extraordinary behavior near the Néel temperature. Approaching Néel temperature, the angular dependence of the emission intensity transforms drastically in shape and width. The rocking curve strongly broadens, splits and acquires a double-peak shape. On the other hand, the energy and time distributions of the emitted radiation strongly depend on the crystal setting in the emission range. In particular, the energy distribution measured in the minimum between the two peaks consist of only one line with width of ∼ 10 neV. Analysis of time distribution showed that in first approximation that line obey squared Lorentzian distribution. The obtained results are in very good agreement with theoretical predictions. The results obtained in this work are highly important to for developing of new synchrotron based techniques with requirements for neV bandwidth of incoming radiation. Acknowelegments Authors are grateful to J.-P. Celse for his help with preparation for the experiment. Also we would like to thank Alexy Bosak and Ilya Sergueev for useful scientific discus sion during paper preparation. 5.3!The 57Fe Synchrotron Mössbauer source at the ESRF! ! ! ! 87! 5.3 The 57Fe Synchrotron Mössbauer source at the ESRF ! Vasily Potapkin,a,b* Aleksandr I. Chumakov,a,c Gennadii V. Smirnov,c Jean-Philippe Celse,a Rudolf Rüffer,a Catherine McCammonb and Leonid Dubrovinskyb ! aEuropean Synchrotron Radiation Facility, BP 220, F-38043 Grenoble, France, bBayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany, and cNational Research Centre "Kurchatov Institute", 123182 Moscow, Russia. *-corresponding author Published in journal of Synchrotron Radiation Vol 19 part 4 p. 559-569 5.3.1 Abstract We describe the design of a 57Fe Synchrotron Mössbauer Source (SMS) for energy-domain Mössbauer spectroscopy using synchrotron radiation at the Nuclear Resonance beamline (ID 18) at the European Synchrotron Radiation Facility (ESRF). The SMS is based on a nuclear resonant monochromator employing pure nuclear reflections of an iron borate (57FeBO3) crystal. The source provides 57Fe resonant radiation at 14.4 keV within a bandwidth of 15 neV, which is tunable in energy over a range of about ±0.6 µeV. In contrast to radioactive sources, the beam of gamma radiation emitted by the SMS is nearly fully resonant and fully polarized, has high brilliance and can be focused to a 10x5 µm2 spot size. Applications include, among others, the study of very small samples under extreme conditions, for example at ultrahigh pressure or combined high pressure and high temperature, and thin films under ultrahigh vacuum. The small cross section of the beam and its high intensity allow for rapid collection of Mössbauer data. For example, the measurement time of a spectrum for a sample in a diamond anvil cell (DAC) at ~100 GPa is around 10 min; whereas such an experiment with a radioactive point source would take more than one week and the data quality would be considerably lower. The SMS is optimized for highest intensity and best energy resolution, which is achieved by collimation of the incident synchrotron radiation beam and thus illumination of the high-quality iron borate crystal within a narrow angular range around an optimal position of the rocking curve. The SMS is permanently located in an optics hutch and is operational immediately after moving it into the incident beam. The SMS is an in-line 5.3!The 57Fe Synchrotron Mössbauer source at the ESRF! ! ! ! 88! monochromator, i.e., the beam emitted by the SMS is directed nearly exactly along the incident synchrotron radiation beam. Thus, the SMS can be easily utilized with all existing sample environments in the experimental hutches of the beamline. Due to a very strong suppression of electronic scattering for pure nuclear reflections (~10-9) SMS operation does not required any gating of the prompt electronic scattering. Thus, SMS can be utilized in any mode of storage ring operation. 5.3.2 Introduction Conventional Mössbauer spectroscopy has been a valuable tool for decades in studying the magnetic and electronic properties of various materials. However, the technique is not well suited to the study of samples with diameters less than ~100 µm. Focusing of radiation from radioactive sources is extremely difficult (Yoshida et al., 2009), and results in a low count rate and a reduction of spectral quality due to an increase of the background. This leads to longer measuring times in order to obtain reasonable quality spectra, and even precludes some studies under extreme conditions, for example the investigation of minerals relevant to the deep Earth at pressures of more than 100 GPa. Thus, the absence of focusing possibilities retards progress in research fields where micron-scale samples are involved, for example, high-pressure geophysics and geochemistry. The time-domain analog of traditional Mössbauer spectroscopy is realized via time-resolved nuclear forward scattering of synchrotron radiation. Synchrotron radiation has very high brilliance and allows for extreme focusing; hence NFS is an excellent tool to study micron-sized samples. However, NFS is not well suited to study highly complex phases, which contain iron in different spin states, valence states and crystallographic sites. The time spectrum of NFS results from an interference of waves coherently scattered by all iron atoms in the sample; therefore it also contains the cross-terms of the scattering amplitudes of iron atoms in different states. Thus, in order to derive the hyperfine parameters for one iron state or site, one needs to fit the hyperfine parameters of all states. In contrast, absorption, by nature is an in-coherent process, results in an energy spectrum, which is linear superposition of constituent components. In most cases this enables the fitting of each iron site or state independently from all others; hence energy-domain Mössbauer spectroscopy is more suitable for studying highly complex phases. Thus for studies of micron-scale 5.3!The 57Fe Synchrotron Mössbauer source at the ESRF! ! ! ! 89! absorbers with complex Mössbauer spectra, the optimum approach would be to employ high-resolution energy-domain Mössbauer spectroscopy in combination with high brilliance and extreme focusing of synchrotron radiation. In short, what is needed is a synchrotron-based source of Mössbauer radiation. Such a source can be designed for 57Fe using the properties of pure nuclear reflections of an iron borate crystal. For reflections of this type electronic diffraction is forbidden, but nuclear diffraction is allowed because of a specific polarization factor of nuclear resonant scattering in the presence of hyperfine interactions (Smirnov et al., 1969). Due to magnetic hyperfine splitting of the 57Fe nuclear levels in the ground and the first excited states, the energy spectrum of the reflected radiation consists of several lines. Under these conditions the nuclear array in a crystal behaves as a multiline radiator. This is, of course, an inconvenient property for performing spectroscopic measurements. However, when the iron borate crystal is heated close to its Néel temperature of 348.35 K, the energy spectrum of the reflected radiation collapses to a single line (Smirnov et al., 1986). Under these conditions, a particular case of hyperfine interaction is realized where magnetic dipole and electric quadrupole hyperfine interactions are strongly mixed (Smirnov et al., 2000). The combined multipath quantum interference in space, energy, and spin domains results in the formation of a pseudo-single-line resonance structure in iron borate (Smirnov et al., 2011), which provides the basis for the creation of a single-line Synchrotron Mössbauer Source (SMS). The spectrum of the emitted radiation has an energy bandwidth close to the natural line width of the Mössbauer resonance. The possibility to develop such a source was demonstrated at the European Synchrotron Radiation Facility (ESRF) in 1997 (Smirnov et al., 1997). Later, the same approach was successfully implemented at SPring-8 (Mitsui et al., 2007a; Mitsui et al., 2007b; Mitsui et al., 2007c; Mitsui et al., 2009). For energy-domain Mössbauer spectroscopy, one needs to scan the energy by changing the relative velocity of the source and the sample using the Doppler effect. In SMS experiments, several methods exist to achieve the Doppler effect. The simplest is to move the sample (Smirnov et al., 1997; Mitsui et al., 2007a; Mitsui et al., 2007b); however this approach is not convenient for most experiments where generally a complicated and/or a heavy sample environment is used. Another possibility is to use an additional silicon crystal before the sample and to achieve the 5.3!The 57Fe Synchrotron Mössbauer source at the ESRF! ! ! ! 96! of its quality. Thus, in order to preserve a good quality of the instrumental function and a high intensity, one needs to have a high quality iron borate single crystal, a small divergence of the incident beam, and a sufficiently accurate rotational stage for precise and stable angular positioning of the crystal. Figure 5.3.4 (a) The rocking curves of pure nuclear (333) reflection (in absolute vertical scale) and of the electronic Umweg reflection (arbitrary vertical scale) of the 57FeBO3 crystal for temperatures in the vicinity of the Néel point. The arrows show the angular positions where the Mössbauer absorption spectra were measured. (b) and (c) The Mössbauer absorption spectra of the single line absorber measured at angular positions 1 and 2, respectively, at a temperature of the iron borate crystal of 348.90 K. Approaching the Néel temperature, the angular width of the rocking curve increases. Accordingly, the dip between the two peaks of the rocking curve where the single line instrumental function can be obtained, becomes more distinct and broad, making it easier to maintain the crystal at the optimal angular position. Furthermore, the broadening of the rocking curve is accompanied by a narrowing of the energy spectrum of radiation reflected by the crystal at the exact Bragg angle (Smirnov et al., 2011). Thus, increasing temperature improves the instrumental function of the SMS. However, this improvement is accompanied by a strong decrease of the intensity of the reflected radiation; therefore a reasonable compromise between the quality of the instrumental function and the count rate has to be found. Under otherwise equivalent conditions, a good quality of the crystal and a proper collimation of the incident beam should allow one to achieve this compromise at higher intensity. 5.3!The 57Fe Synchrotron Mössbauer source at the ESRF! ! ! ! 97! Fig. 5.3.4a shows the rocking curves of the (333) reflection for two different temperatures in the vicinity of the Néel point. The first rocking curve (green circles) is obtained at the temperature 348.90 K where an optimal compromise between the energy width of the instrumental function and the intensity of the reflected beam is achieved. The second rocking curve shown in Fig. 5.3.4a (blue triangles) is measured at a temperature 0.05 K above the previous one. This small change of temperature increases the width of the rocking curve from 33 µrad to 37 µrad, which slightly relaxes the requirements for collimation of the beam, quality of the crystal, and angular stability. However, this lowering of the tolerance to angular distortions is accompanied by ~40% loss of intensity. Thus, even small improvements of the beam collimation and crystal quality could provide a tremendous gain in the efficiency of the SMS. The instrumental functions of the SMS were evaluated using a single line K2Mg57Fe(CN)6 absorber with an area density of the resonant 57Fe isotope of 0.50 mg/cm2 and a line width of 2.1 Γ0 (Γ0 = 0.097 mm/s). Fig. 5.3. 4b shows the absorption spectrum measured when the iron borate was set to the angular position at the centre of the dip on the rocking curve (Fig. 5.3.4a). The obtained line width of 4.3 Γ0 testifies that the line width of the instrumental function at the optimal angular position is about 2.2 Γ0 (after absorber width deconvolution). At the angular positions of both peaks of the rocking curve, the instrumental function acquires undesirable satellite peaks. The same effect happens when the iron borate crystal is not of sufficient quality: the double-peak structure is washed out and the instrumental function measured at the exact Bragg angle acquires satellite peaks due to the effective angular averaging. Under these conditions, a single line absorption spectrum can be obtained at the “backup” angular position at the highangle slope of the rocking curve (Fig. 5.3.4a). However, the energy width of the instrumental function at the backup position is larger: the measured line width of 6.7 Γ0 (Fig. 5.3.4c) shows that the line width of the instrumental function is about 4.6 Γ0 (after absorber width deconvolution). 5.3.3.6. Umweg reflections Even if a particular reflection is forbidden for electronic diffraction, the X-ray beam may find a way for multiple scattering in the crystal lattice. The X-rays may 5.3!The 57Fe Synchrotron Mössbauer source at the ESRF! ! ! ! 98! experience diffraction on two or more sets of crystalline planes so that the beam is reflected in the same direction as if the reflection would be allowed. Such “bypass” reflections are called Umweg reflections (Renninger, 1937). These reflections are characterized by a broad energy bandwidth typical for electronic diffraction. The Umweg reflections may add intense non-resonant radiation to the reflected beam, thus creating a strong non-resonant background in the Mössbauer spectrum. In order to avoid this background, one needs to escape Bragg conditions for one or more sets of planes involved in Umweg reflections while preserving the Bragg condition for pure nuclear reflection. This can be achieved by rotating the iron borate crystal around the axis of the scattering vector, i.e., by scanning the azimuthal angle of the crystal. For the crystal of iron borate, the azimuthal dependence of the (NNN) Umweg reflections has six-fold symmetry and 12 symmetry points, which correspond to the and crystallographic directions. Fig. 5.3.5 shows the azimuthal dependence of the Umweg reflections in the vicinity of the direction for the (111) pure nuclear reflection. For this measurement, the crystal was heated well above its Néel temperature, where nuclear scattering was completely suppressed and the monitored intensity originated only from electronic scattering of the Umweg reflections. Near the symmetry point, the tails of the neighboring Umweg reflections fortunately cancel each other out (Fig. 5.3.5). Thus, at this particular azimuthal position the nonresonant background in the absorption Mössbauer spectra taken with the SMS is suppressed almost completely. The intensity of prompt electronic scattering in Umweg reflections is much higher than the intensity of resonant scattering of pure nuclear reflections and may be used to advantage. In particular, with Umweg reflections it is much easier to align the sample inside the DAC to the beam and to adjust focusing optics. 5.3!The 57Fe Synchrotron Mössbauer source at the ESRF! ! ! ! 99! Figure 5.3.5 The azimuthal dependence of Umweg reflections for the pure nuclear (111) reflection of the iron borate crystal in the vicinity of the azimuthal position along the € [101] crystallographic direction. The data were taken at 353.15 K. 5.3.3.7. Energy modulation. The furnace with the iron borate crystal is mounted on a standard Mössbauer transducer, which moves the crystal in the plane parallel to the crystal surface. There are several approaches to understand how the energy modulation of the reflected beam occurs: considering the SMS as a monochromator, one notes that the motion of the crystal along its surface does not change the energy of the X rays. Indeed the utilized pure nuclear (NNN) reflections are symmetric. Therefore, the incoming and outgoing beams form equal angles with the plane of motion and the Doppler shift in frequency of the incoming radiation is compensated by exactly the opposite Doppler shift of outgoing radiation. Thus the frequency of the incoming radiation is transmitted to the same frequency of the outgoing radiation. In the case of electronic scattering the atomic scattering amplitude is constant within the range of the Doppler shift. However, nuclei can only be excited by the frequency equal to their own resonant frequency. By moving nuclei with different velocities, one can effectively change the resonant frequency of the nuclei. The bandwidth of the resonant energy is much less than the range of the Doppler shifts. On the other hand, the bandwidth of incoming radiation is larger than the Doppler shift range. Therefore, nuclei being moved with different velocities effectively select from the incoming beam the 5.3!The 57Fe Synchrotron Mössbauer source at the ESRF! ! ! ! 100! radiation components with different frequencies. In this way the energy modulation of the reflected radiation is achieved. Alternatively, one may consider the SMS as a coherent radiator of nuclear gamma rays. The 57Fe nuclei of the iron borate crystal are excited by the white spectrum of incident radiation for any velocity of the moving crystal. After excitation, the nuclei emit nuclear gamma radiation. The energy of this radiation is equal to the energy of the nuclear resonant transition. Similar to a conventional radioactive source, the translational motion of the iron borate crystal case a Doppler shift and leads to the energy modulation of the emitted radiation. In contrast to other approaches to obtain the Doppler shift (Mitsui et al., 2007c), moving the iron borate crystal along the plane of the crystal surface has an important advantage: this motion does not cause a spatial displacement of the reflected beam. Therefore, the energy modulation does not compromise focusing and does not shift the beam relative to the sample (Mitsui et al., 2007d). From the point of view of oscillatory dynamics, ideally the furnace should be located at the centre of inertia of the moving frame of the transducer. However, this requirement is difficult to combine with the diffraction geometry. To allow for diffraction, the furnace is attached to the moving frame above the transducer (Fig. 5.3.6). Under such conditions the frame is obviously not balanced, i.e., its centre of mass is shifted upwards from the axis of the transducer. This shift leads to the appearance of an inertial force torque. Under these conditions, the translation movement of the frame would be accompanied by rocking motions, which would result in a variation of the angle between the incident radiation and the crystal during the crystal motion. This would case (i) broadening of the instrumental function and (ii) oscillations of the intensity of the reflected beam. Obviously, the quality of Mössbauer spectra with such a source would be significantly lower; firstly, because of diminished energy resolution and secondly, because the base line (i.e., the Mössbauer spectrum collected without sample) would no longer be flat. In order to avoid these drawbacks, the frame is balanced by a counterweight which is mounted on the bottom side of the frame below the transducer. The distances from the axis of the transducer to the counterweight and to the furnace are equal. For adjustment of the mass of the counterweight, the furnace is exchanged by a silicon crystal with a mass equal to the mass of the furnace. The angular stability of the frame during its translational motion is evaluated by monitoring the intensity of 5.3!The 57Fe Synchrotron Mössbauer source at the ESRF! ! ! ! 101! the beam reflected by the silicon crystal as a function of the velocity of the frame. With a properly adjusted mass of the counterweight, the measured base line is straight. In order to reach the highest sensitivity of the adjustment, the base line is measured at the half-maximum position of the rocking curve of the silicon crystal. After adjustment of the counterweight, the estimated residual variation of the angular position of the frame is about 1 µrad. This is 10 times smaller than the width of the angular range near the exact Bragg angle of the pure nuclear (333) reflection of the iron borate crystal where the single-line spectrum of the instrumental function can be achieved. 5.3.3.8. Mechanics of the SMS In order to operate the SMS, the following rotations and translations of the iron borate crystal are required: firstly, one needs a very accurate rotation stage to adjust the Bragg angle of the crystal. Secondly, in order to avoid Umweg reflections, an azimuthal rotation is needed. Finally, the tilt rotation of the crystal is needed to keep the reflected beam in the vertical plane. All these rotational stages should be precise and stable. In addition, two translations (a horizontal and a vertical one) are needed to adjust the crystal position relative to the incident synchrotron radiation beam. Fig. 5.3.6 shows a schematic view of the mechanics of the SMS. The bottom part is a large 2-circle segment (Huber 2-circle segments 5203.80), and the iron borate crystal is located at the centre of rotation of this segment. One circle is utilized for Bragg angle adjustments and is equipped with a 20:1 gear box. The accuracy of angular positioning of this rotation stage is about 0.5 µrad. The other circle is utilized for tilt angle adjustments; the accuracy of angular positioning of this stage is about 2 µrad. The vertical translation stage (Huber z-stage 5103.A20-90) is mounted on the 2circle segment; the accuracy of linear positioning provided by this stage is better than 5 µm. On the vertical translation stage a wedge is mounted which supports the stage of azimuthal rotation. The wedge angle is equal to the Bragg angle of the chosen pure nuclear reflection of the iron borate crystal. The 1-circle goniometer (Huber 1-circle goniometer 409) is used for azimuthal rotation and is equipped with a 10:1 gear box. The accuracy of angular positioning provided by this rotation stage is about 5 µrad. The Mössbauer transducer (Wissel MVT-1000) equipped with the frame and the 5.3!The 57Fe Synchrotron Mössbauer source at the ESRF! ! ! ! 102! furnace is mounted on the azimuthal stage. The entire mechanics are mounted on a horizontal translation stage which is used to adjust the horizontal position of the crystal relative to the beam and to move the entire setup in and out of the beam. The accuracy of linear positioning provided by this stage is about 5 µm. Figure 5.3.6 The mechanics of the Synchrotron Mössbauer Source: (1) furnace; (2) transducer; (3) 1-circle goniometer for azimuthal rotation; (4) vertical translation stage; (5) 2-circle segment for Bragg and tilt rotations; (6) counterweight of the frame; (7) wedge. 5.3.4 Optical scheme 5.3.4.1. In-line setup Apart from the iron borate crystal, the optical scheme of the SMS includes two additional optical elements. The first optical element is an additional monochromator. The beam after the high-heat-load monochromator is so intense that it may produce a temperature gradient over the surface of the iron borate crystal. In order to avoid the temperature gradient, an additional monochromator is needed whose purpose is to decrease the energy bandwidth of the beam incident to the iron borate crystal and thus 5.3!The 57Fe Synchrotron Mössbauer source at the ESRF! ! ! ! 103! to decrease the heat load. For this purpose the bandwidth can be chosen to be only moderately (not extremely) small. Under these conditions, the additional monochromator can be relatively easily optimized for an essentially high throughput. Therefore, the inclusion of the additional monochromator essentially does not decrease the intensity of the beam provided by the source. The second additional optical element is a deflector. The deflector is needed in order to make the SMS an inline monochromator. The deflector rotates the beam incident to the iron borate crystal in the vertical plane so that the beam emitted by the crystal is directed nearly horizontally, almost exactly along the synchrotron radiation beam from the high-heatload monochromator. The choice of the additional monochromator and deflector depends on the utilized pure nuclear reflection. In this paper, we describe the in-line optical schemes for the (111) and (333) pure nuclear reflections of the iron borate crystal. Figure 5.3.7 The optical scheme for a high-pressure experiment with DAC using the Synchrotron Mössbauer Source based on the (333) pure nuclear reflection. U – undulator; HHLM – high-heat-load monochromator; CRL – compound refractive lens; SMS – the Synchrotron Mössbauer Source: HRM – high resolution monochromator, Def – Si (311) deflector, IB –the iron borate crystal inside the furnace with the four magnets and mounted on the Mössbauer transducer; KBM - Kirkpatrick-Baez mirrors; DAC – diamond anvil cell; D – avalanche photo diode detector. (333) reflection: The pure nuclear (333) reflection of iron borate is convenient for the deflector reflection (311) of silicon. For 14.4 keV radiation, the Bragg angle of the silicon (311) reflection is 15.23 degree and nearly matches that of the iron borate (333) reflection (15.49 degree). Thus, the angle between the beam from the SMS and the horizontal plane is only 0.52 degrees. This enables the beam to reach all downstream experimental hutches and to use all sample environments available at the beamline. This deflector can be easily combined with any additional in-line monochromator. This is a significant advantage of the SMS based on the (333) pure nuclear reflection of iron borate. In this work we use an in-line high-resolution 5.3!The 57Fe Synchrotron Mössbauer source at the ESRF! ! ! ! 104! monochromator (HRM) with a bandwidth of ~15meV. It is a double crystal monochromator with two asymmetric (975) reflections of silicon crystals. Fig. 5.3.7 shows a typical optical scheme for a high-pressure experiment with a DAC using SMS based on the (333) pure nuclear reflection. The undulator (U) is the source of the synchrotron radiation beam. The high-heat-load monochromator (HHLM) decreases the energy bandwidth of the beam to ~2 eV, and the compound refractive lens (CRL) is used to decrease the divergence of the beam incident to the SMS. The high-resolution monochromator (HRM) decreases the energy bandwidth of the beam further to ~15 meV. The deflector (Def) directs the beam to the iron borate crystal (IB) where the final monochromatization within ~10 neV occurs. The beam from the SMS is focused by the Kirkpatrick-Baez mirrors (KBM) on the sample located in the DAC. The gamma radiation transmitted through the sample is monitored by the avalanche photo diode detector (D). Figure 5.3.8 The optical scheme for a high-pressure experiment with a DAC using Synchrotron Mössbauer Source (SMS) based on the (111) pure nuclear reflection. U - undulator; HHLM – high-heat-load monochromator; CRL – compound refractive lens; SMS – the Synchrotron Mössbauer Source: MRM – deflector and medium resolution monochromator with ~100meV bandwidth, IB – the iron borate crystal inside the furnace equipped with four magnets and mounted on the Mössbauer transducer; KBM – Kirkpatrick-Baez mirrors; DAC – diamond anvil cell; D – avalanche photo diode (APD) detector. (111) reflection: Another reasonable option for the SMS is the (111) pure nuclear reflection. The angular width of this reflection is three times larger than that of the (333) one. Therefore, the requirements for the angular stability, crystal quality, and collimation of the incident beam are more relaxed. As described above, under otherwise equivalent conditions this helps to achieve a narrower instrumental function and higher intensity. However, the Bragg angle of the (111) reflection is three times smaller than that of the (333) one. Therefore, the area illuminated by the incident beam on the surface of the iron borate crystal is three times larger. Thus, higher quality iron borate crystals are required. Furthermore, for the (111) reflection of iron borate there is no convenient reflection of silicon for the deflector with a sufficiently 5.3!The 57Fe Synchrotron Mössbauer source at the ESRF! ! ! ! 105! close Bragg angle. The deflector for the (111) reflection can be obtained using two reflections: Si(422) and Si(531). The difference between the Bragg angles of these reflections is 5.111 degrees while the Bragg angle of the iron borate (111) reflection is 5.108 degrees. Thus, the beam reflected by the {Si(422) + Si(531)} deflector and the iron borate (111) reflection deviates from the horizontal plane by an angle less than 100 µrad. This makes the SMS based on the (111) pure nuclear reflection an in-line monochromator. This deflector also decreases the energy bandwidth to ~100 meV. Thus, it acts both as a deflector and an additional monochromator. The advantage of this setup is that one may combine the SMS experiment with any other technique such as inelastic scattering or diffraction: the beam after the SMS based on the (111) pure nuclear reflection matches the path of the incident synchrotron radiation beam within an accuracy of 100 µrad. Thus, one can move the SMS out of the beam and exchange it with any other in-line monochromator without readjusting the focusing optics or sample position. Fig. 5.3.8 shows the typical optical scheme for a high-pressure experiment with a DAC using the SMS based on the (111) pure nuclear reflection. The medium resolution monochromator (MRM) with silicon reflections Si(422) and Si(531) serves as both an additional monochromator and a deflector. All elements before and after the SMS are the same as for the setup based on the (333) pure nuclear reflection. Although most of the problems encountered during source development have been solved, there is one unexpected effect that occurs, namely, bending of the initially flat iron borate crystal. The bending appears after about one week of operation and only when the crystal is heated up to its Néel temperature. Although the initial crystal state recovers after a period of relaxation, the bending effect compromises system stability and requires further investigation. Figure 5.3.9 The profiles and sizes of the focal spot in (a) horizontal and (b) vertical directions. The arrows show the full widths at the half maximum of the profiles. 5.4!No spin transition in ferric iron in the lower mantle! ! ! ! 112! 5.4 No spin transition of ferric iron in the lower mantle V.Potapkin1,2, C.McCammon2*, L.Dubrovinsky2, K.Glazyrin2, A. Kantor2, I. Kupenko2, C. Prescher2, R. Sinmyo2, G. Smirnov3, S. Popov3, A. Chumakov1,3 and R.Rüffer1 1European Synchrotron Radiation Facility, BP 220, F-38043 Grenoble, France 2Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany 3National Research Center "Kurchatov Institute", 123182 Moscow, Russia *-corresponding author published in Nature Communication (2013) 4 1427 5.4.1. Abstract Iron has the ability to adopt different electronic configurations (spin states) in the dominant lower mantle phase, magnesium silicate perovskite, which can significantly influence mantle properties and dynamics. Previous studies have suggested that ferric iron (which constitutes at least half of the iron in lower mantle silicate perovskite (McCammon, 1997; Frost et al., 2004;McCammon, 2005)) undergoes a high-spin to low-spin transition (Zhang et al., 2006; Stackhouse et al., 2007; Catalli et al., 2010; Hsu et al. 2011; Catalli et al., 2011) which has been suggested to be the cause of some observed seismic velocity anomalies (Catalli et al., 2011;Hsu et al. 2011). Here we apply a new synchrotron-based method of Mössbauer spectroscopy to four different compositions of lower mantle silicate perovskite at high pressure and high temperature and demonstrate unambiguously that Fe3+ remains in the high-spin state at conditions throughout the lower mantle. Our results rule out the possibility that lateral heterogeneities of geophysical parameters in the mid-lower mantle (bulk sound speed, electrical conductivity) can be explained by iron spin transitions in silicate perovskite, and advocate instead a compositional or thermal variation. 5.4.2. Introduction The structure of iron-containing magnesium aluminum silicate perovskite (hereafter referred to as FeAlPv) contains two sites, a large 8-12 coordinated site (“A”) which is primarily occupied by Mg2+ and Fe2+, and a smaller octahedral site (“B”) which is dominantly occupied by Si4+ and to a lesser extent by Al3+. Although there is disagreement in Fe3+ spin transition pressures reported by previous studies, broadly a high-spin (HS) (five unpaired d electrons) to low-spin (LS) (one unpaired d 5.4!No spin transition in ferric iron in the lower mantle! ! ! ! 113! electron) transition is predicted when Fe3+ occupies the B-site, while Fe3+A is predicted to remain in the high-spin state at all pressures throughout the lower mantle (Zhang et al., 2006; Stackhouse et al., 2007; Catalli et al., 2010;Hsu et al. 2011; Catalli et al., 2011). Even though Fe3+ is predicted to occupy exclusively the A-site in lower mantle compositions of FeAlPv (Vanpeteghem et al., 2006), a HS-LS transition of Fe3+ was reported to occur in FeAlPv at high pressure, potentially due to exchange of Fe3+ from the Ato the B-site (Catalli et al., 2011, Fujino et al., 2012). Up to now, however, only methods which do not provide an unambiguous interpretation of the data have been used, so we have developed a new method which enables individual spin and valence states to be identified, which provides a clear answer to the question of whether Fe3+ undergoes a HS-LS transition in FeAlPv. 5.4.3. Experimental Methods 5.4.3.I. Introduction Detecting spin transitions of Fe3+ in FeAlPv presents a significant challenge. Xray emission spectroscopy provides information on the bulk spin number, but cannot separate individual contributions. In contrast energy domain 57Fe Mössbauer spectroscopy generally enables an unambiguous resolution of all hyperfine parameters which can be used to infer spin states; however high pressure measurements using conventional radioactive point sources require extremely long counting times (generally more than one week per spectrum). Third generation synchrotron sources offer a solution in the form of time-domain Mössbauer spectroscopy (i.e., nuclear forward scattering); however this method is not well suited to materials with a large number of components (such as FeAlPv) due to the non-uniqueness of fitting models. To solve this problem, we have developed an energy-domain synchrotron Mössbauer source (SMS), which offers a number of advantages: high flux, a beam diameter of a few microns and zero background. SMS allows for rapid measurement of energydomain Mössbauer spectra under extreme conditions with a quality generally sufficient to unambiguously deconvolute even highly complex spectra (further details of the method are given in the Supplementary Information). In order to investigate the spin state of iron in lower mantle silicate perovskite, we studied four different silicate perovskite samples: Mg0.63Fe0.37Si0.63Al0.37O3+δ sample #1 (~80% Fe3+/ΣFe); Mg0.63Fe0.37Si0.63Al0.37O3+δ sample #2 (~70% Fe3+/ΣFe); 5.4!No spin transition in ferric iron in the lower mantle! ! ! ! 114! Mg0.78Fe0.2Al0.05Si0.97O3+δ (~50% Fe3+/ΣFe); Mg0.94Fe0.06SiO3+δ (~20% Fe3+/ΣFe). We collected SMS spectra at room temperature and pressures up to 122 GPa using diamond anvil cells, with or without laser annealing of the samples. Details of sample synthesis and experiment methodology are given in the Supplementary Information. 5.4.3.II. Methods 57Fe Synchrotron Mössbauer source (SMS) spectra were collected on beamline ID18 at the European Synchrotron Radiation Facility (ESRF) during operation in uniform mode (7/8 filling) with the beam focused to roughly 10×10 µm2 using a Kirkpatrick–Baez mirror. Further details of the SMS method are given in the Supplementary Information. The velocity scale was calibrated relative to α-Fe foil, and spectra were collected over 10-60 min each. Spectra were fitted using the program MossA (Prescher et al., 2012). The dimensionless effective Mössbauer thicknesses were approximately 40 and 20 for the Mg0.63Fe0.37Si0.63Al0.37O3 perovskite samples #1 and #2, and 8 and 3 for the Mg0.78Fe0.2Al0.05Si0.97O3 and Mg0.94Fe0.06SiO3 perovskite samples, respectively. Further details of sample synthesis and experimental methodology are given in the Supplementary Information. 5.4.4. Results and Discussion SMS spectra of Mg0.63Fe0.37Si0.63Al0.37O3 perovskite, which contains iron dominantly as Fe3+, are extremely well resolved (Fig. 5.4.1). We fit the data to three quadrupole doublets, one assigned to Fe3+ and two assigned to Fe2+ (high QS and low QS) based on their centre shifts. Visually there appears to be no change to the Fe3+ doublet over the entire pressure range (Fig. 5.4.1 and Supplementary Fig. 5.4.S4). The hyperfine parameters (centre shift [CS] and quadrupole splitting [QS]) in all samples studied are the same within experimental error, and there is no change in Fe3+/ΣFe for individual samples over the entire pressure range of the experiment (Fig. 5.4.2). The hyperfine parameters of the Fe3+ doublet are consistent with the high-spin state (Gütlich et al., 2011), and their smooth variation with pressure indicates that Fe3+ does not undergo any spin transitions within the entire pressure range. Notably the QS value reported for low-spin Fe3+ from both experimental (Catalli et al., 2010; Catalli et al., 2011) and theoretical (Hsu et al., 2011) studies (red dashed line, Fig. 5.4.2a) is more than twice our observed values. 5.4!No spin transition in ferric iron in the lower mantle! ! ! ! 115! The hyperfine parameters of the low QS Fe2+ doublet (blue doublet in Fig. 5.4.1) corresponds to the high-spin state (McCammon et al., 2008), while the doublet with high quadrupole splitting (black doublet in Fig. 1) corresponds either to intermediatespin (IS) Fe2+ (McCammon et al., 2008) or a distortion of the site occupied by highspin Fe2+ (Hsu et al., 2011). Irrespective of the interpretation of the Fe2+ spin state, our conclusions regarding the absence of a spin transition in Fe3+ remain the same. Figure. 5.4.1: SMS spectra of Mg0.63Fe0.37Si0.63Al0.37O3 perovskite sample #1 at room temperature showing their evolution with pressure (a) 2.5 GPa; (b) 37.9 GPa; (c) 67 GPa; (d) 93 GPa. The data were fit to one Fe3+ doublet (red) and two Fe2+ doublets (blue and black), and the fit residual is shown below each spectrum. Area asymmetry is due to preferred orientation of the sample, and the velocity scale is given relative to α-iron. Single-crystal X-ray refinements (Glazyrin et al., 2011) and crystal chemical calculations (see Supplementary Information) have shown that Fe3+ occupies only the A-site of the perovskite structure in our samples; hence our data demonstrate that Fe3+A does not undergo a high-spin to low-spin transition up to at least 122 GPa. This conclusion is in agreement with results from experimental (Catalli et al., 2010; Catalli 5.4!No spin transition in ferric iron in the lower mantle! ! ! ! 116! et al., 2011) and theoretical (Zhang et al., 2006; Stackhouse et al., 2007;Hsu et al. 2011) studies. However several studies (Catalli et al., 2010; Catalli et al., 2011; Fujino et al., 2012) have suggested that the reduced volume of LS Fe3+B could lead to a redistribution of Fe3+ from the Ato the B-site in the perovskite structure within the lower mantle. To test this hypothesis, we laser annealed our samples at a number of pressures and collected SMS spectra both during and after heating. Visually SMS spectra collected after laser annealing showed no change to those taken before heating (Supplementary Fig. 5.4.S5) and hyperfine parameters remain unchanged (star symbols; Fig. 5.4.2). We therefore conclude that there is no exchange of Fe3+ between the Aand the B-site at lower mantle pressures and temperatures, in agreement with the results of a complementary study using high-pressure single-crystal X-ray diffraction with laser heating undertaken on the same composition as two of our samples (Glazyrin et al., 2011). The coupled substitution Mg2+A + Si4+B ↔ Fe3+A + Al3+B is considered to be the dominant mechanism for incorporation of Fe3+ into FeAlPv in the lower mantle (Vanpeteghem et al., 2006; Brodholt et al., 2000; Saikia et al., 2009); hence Fe3+ is expected to only occupy the A-site in the perovskite structure for lower mantle compositions. Electrical conductivity measurements of single-phase Mg0.9Fe0.1SiO3 perovskite and a pyrolite mantle assemblage all show a decrease in conductivity above 50 GPa (Ohta et al., 2008; Ohta et al., 2010a; Ohta et al., 2010b), comparable to the electrical conductivity decrease caused by HS-LS spin crossover of Fe2+ in (Mg,Fe)O (Yoshino et al.,2011). The drop in silicate perovskite conductivity has been attributed to a HS-LS transition of Fe3+ (Ohta et al., 2008; Ohta et al., 2010a; Ohta et al., 2010b); however the small proportion of Fe3+ in the Mg0.9Fe0.1SiO3 perovskite sample (~ 10%), the results from this study and a companion study (Glazyrin et al., 2012) that there is no shift of Fe3+ from the A-site to the B-site at lower mantle conditions, and the overwhelming evidence that no Fe3+ spin transition occurs when Fe3+ occupies the A-site in FeAlPv implies that the drop in electrical conductivity cannot be attributed to a spin transition of Fe3+. A more likely explanation is the pressure-induced HS-IS transition of Fe2+ in FeAlPv observed using X-ray emission spectroscopy (Badro et al., 2004) and nuclear resonance methods (McCammon et al., 2004; Lin et al., 2004). 5.4!No spin transition in ferric iron in the lower mantle! ! ! ! 117! Figure 5.4.2: Pressure variation of FeAlPv hyperfine parameters (centre shift [CS] and quadrupole splitting [QS]) (a) high-spin Fe3+; (b) high QS Fe2+. Sample data are indicated as follows: Mg0.63Fe0.37Si0.63Al0.37O3 perovskite sample #1 (red and blue circles); Mg0.63Fe0.37Si0.63Al0.37O3 perovskite sample #2 (orange and blue squares); Mg0.78Fe0.2Al0.05Si0.97O3 perovskite (brown and dark blue circles); Mg0.94Fe0.06SiO3 perovskite (purple and green circles). The expected quadrupole splitting for low-spin Fe3+ is shown as a horizontal red dashed line. (c) Pressure variation of Fe3+/ΣFe as determined from the relative areas: perovskite sample #1 (red circles); Mg0.63Fe0.37Si0.63Al0.37O3 perovskite sample #2 (orange squares); Mg0.63Fe0.37Si0.63Al0.37O3 perovskite (brown circles); Mg0.94Fe0.06SiO3 perovskite (purple circles). Horizontal lines are guides for the eye. Values obtained after laser heating of Mg0.63Fe0.37Si0.63Al0.37O3 perovskite sample #2 and Mg0.78Fe0.2Al0.05Si0.97O3 perovskite are indicated by grey stars. In all cases, parameters remained unchanged from those before heating within experimental error, demonstrating that there is no high-spin to low-spin transition in Fe3+ at the pressure-temperature conditions of the lower mantle. Electromagnetic induction data offer an important complement to seismic data with respect to lower mantle modeling, since the former are more sensitive to 5.4!No spin transition in ferric iron in the lower mantle! ! ! ! 118! temperature and iron content, while the latter better constrain the mineralogy (Verhoeven et al, 2009). Laboratory electrical conductivity data of the relevant minerals are an important component of the approach, and it is crucial that they are representative of behavior in the depth range of interest. Earlier laboratory-based electrical conductivity models incorporating FeAlPv conductivity values at the top of the lower mantle reported that electrical conductivity in the lower mantle is relatively homogeneous (Xu et al, 2000), while a more recent model incorporating the latest experimental conductivity values found a decrease in electrical conductivity between 1300 and 1800 km depth (Ohta et al., 2010). Indeed a recent one-dimensional inversion of 32 years of geomagnetic data shows a decrease in conductivity below 1400 km, and a three-dimensional inversion of the same dataset shows variations of more than one order of magnitude in the mid part of the lower mantle, some of which are correlated with fast and slow regions of seismic tomography models (Tarits et al., 2010). Based on the results of this study we can rule out the possibility that seismic anomalies in the mid-lower mantle region are caused by a Fe3+ spin transition in FeAlPv, and compressibility studies have already shown that there is no pressuredinduced change to elasticity in this depth range for Fe2+-rich silicate perovskite (McCammon et al., 2008; Saikia et al., 2009; Lundin et al., 2008). Such variations in electrical conductivity are more likely caused by compositional or temperature heterogeneity, but may also be due to minor phases such as water or carbonates (Tarits et al., 2010). A final resolution of the controversy surrounding iron spin transitions in FeAlPv through Mössbauer spectroscopy (this work) and single crystal X-ray diffraction (Glazyrin et al., 2012) provides an important step in developing quantitative models for joint inversion of electromagnetic and seismic data that place improved constraints on lower mantle mineralogy, composition and thermal state. Acknowledgements We acknowledge the European Synchrotron Radiation Facility for provision of synchrotron radiation facilities (ID18) and we would like to thank Jean-Philippe Celse for additional technical assistance and Ilya Sergueev for help with data analysis software. The project was partly supported by funds from the German Science Foundation (DFG) Priority Programme SPP1236, the PROCOPE exchange programme, and the German Federal Ministry for Education and Research (BMBF). 5.4!No spin transition in ferric iron in the lower mantle! ! ! ! 119! 5.4.5. Supplementary Information 5.4.5.I. Synchrotron Mössbauer source Energy-domain Mössbauer spectroscopy provides direct access to hyperfine parameters from which iron valence and spin state can be determined. It has a large advantage compared to nuclear forward scattering (NFS) and X-ray emission spectroscopy (XES) in that each contribution occupies a nearly unique energy range which generally enables an unambiguous resolution of all components. In contrast, NFS and XES provide bulk information which combines the effects of all contributions. This advantage of Mössbauer spectroscopy makes it an ideal tool to study systems where iron exists in different spin and valence states, and different crystallographic positions. Conventional energy-domain Mössbauer spectroscopy uses radioactive sources whose brilliance is very low; hence high-pressure studies using diamond anvil cells (DACs) require long measuring times since beam focusing is not possible in a laboratory setting. This reduces the quality of the results due to pressure gradients, increased background, and restricts the maximum pressure at which measurements are possible. These problems can be solved by combining the advantages of highbrilliance third generation synchrotrons (high flux, extreme focusing of the beam) with the resolution of energy-domain Mössbauer spectroscopy to produce a synchrotron source of Mössbauer radiation. A synchrotron Mössbauer source (SMS) provides a high-brilliance beam of synchrotron radiation with an energy bandwidth of ~15 neV. In addition, SMS has several further properties that a radioactive source does not possess. The SMS beam is polarized up to 99%, and it consists 100% of recoilless radiation with zero background. These properties enable rapid and precise measurements of Mössbauer spectra of samples under extreme conditions. The possibility to develop such a source was first demonstrated at the nuclear resonance beamline ID18 (Rüffer & Chumakov 1996) at the European Synchrotron Radiation Facility (ESRF) in 1997 (Smirnov et al., 1997). 5.4!No spin transition in ferric iron in the lower mantle! ! ! ! 120! Figure 5.4.S1 Optical scheme for SMS experiment for high-pressure studies. U - undulator; HHLM – high-heat load monochromator; HRM – high resolution monochromator; SMS: Si – Si (311) crystal; IB – enclosed furnace (mounted on a velocity transducer) with the iron borate crystal inside; KBM - Kirkpatrick-Baez mirrors; DAC – diamond anvil cell; D – avalanche photo diode detector. Fig. 5.4.S1 shows the experimental setup. The synchrotron beam is emitted the undulator (U), passes through high-heat load monochromator (HHLM) with energy bandwidth of ~2 eV, and a high-resolution monochromator (HRM) with energy bandwidth of ~15 meV. This cascade of monochromators is required to decrease the heat load on the iron borate crystal in order to prevent a temperature gradient. The SMS itself is a monochromator composed of two crystals. The key element of the source is an iron borate crystal (FeBO3) enriched in the 57Fe isotope. The crystal is used in (333) pure nuclear reflection. For such reflection, electronic diffraction is forbidden, while nuclear diffraction is allowed. The crystal is placed in an external magnetic field of ~110 Oe and heated close to its Néel temperature of ~75.65°C. Under these conditions the hyperfine magnetic structure collapses to a single line. The crystal therefore reflects synchrotron radiation within an energy bandwidth of ~20 neV. Further details of the physics of the process are given in (Smirnov et al., 2011). The furnace is enclosed and special arrangements of the magnets are used in order to ensure homogenous heating and magnetic field for stable operation. The furnace with the crystal inside is mounted on a velocity transducer in order to modulate the energy. The purpose of the Si (311) crystal is to direct the beam exiting the SMS in a direction parallel to the incoming synchrotron beam, which allows for a more convenient installation of different types of sample environment such as DAC, cryostat, furnace, etc. The SMS is followed by a focusing mirror (KBM), which allows focusing of the 5.4!No spin transition in ferric iron in the lower mantle! ! ! ! 121! synchrotron beam to a roughly 10 x 10 µm spot size on the sample located in the DAC. The transmitted γ-quanta are monitored by an avalanche photo diode detector. During the SMS experiment the linewidth of the source is controlled before and after each sample measurement using K2Mg57Fe(CN)6, whose Mössbauer spectrum consists of a single line (Fig. 5.4.S2). The velocity scale is calibrated using 25 µm thick natural α-iron foil (Fig. 5.4.S3). Figure 5.4.S2 SMS spectrum of K2Mg57Fe(CN)6 used to monitor the source linewidth. 5.5!Portable Double-Sided Laser-Heating System for SMS and Single! Crystal Diffraction Experiments with DACs! ! ! 128! 5.5. Portable Double-Sided Laser-Heating System for Energy-Domain Mössbauer Spectroscopy at Synchrotron and Single Crystal Diffraction Experiments with Diamond Anvil Cells I. Kupenko1,*, L. Dubrovinsky1, N. Dubrovinskaia2, C. McCammon1, K. Glazyrin1, E. Bykova1,2, T. Boffa Ballaran1, R. Sinmyo1, A.I. Chumakov3, V. Potapkin1,3, A. Kantor1,3, R. Rüffer3, M. Hanfland3, W. Crichton3, M. Merlini4 1Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany 2Material Physics and Technology at Extreme Conditions, Laboratory of Crystallography, University of Bayreuth, D-95440 Bayreuth, Germany 3European Synchrotron Radiation Facility, BP 220, F-38043 Grenoble, France 4Dipartimento di Scienze della Terra, Università degli Studi di Milano, Via Botticelli 23, 20133 Milano (Italy) *-coresponding author published in Review Scientific Instruments (2012) 83 124501 5.5.1. Abstract Diamond anvil cell (DAC) technique coupled with laser heating is a major method to study materials at static multimegabar pressures and at high temperatures. Recent progress in experimental techniques, especially in high-pressure single crystal X-ray diffraction, requires portable laser heating systems, which may heat and move the DAC during data collection. We have developed a double-sided laser heating system for DACs which can be mounted on a ~0.1 m2 area and has a weight of ~12 kg. The system is easily transferable between different in-house or synchrotron facilities and can be assembled and set up during a few hours. The system was successfully tested at the ID09a and ID18 beamlines of the European Synchrotron Radiation Facility (ESRF). We demonstrate application of the system on example of a single crystal Xray diffraction investigation of (Mg0.87,Fe3+0.09,Fe2+0.04)(Si0.89,Al0.11)O3 silicate perovskite and energy-domain Mössbauer spectroscopy study of (Mg0.8Fe0.2)O ferropericlase using recently developed Synchrotron Mössbauer Source at high pressures and temperatures. 5.5.2. Introduction Studies of materials at extreme pressure and temperature conditions are very important for natural sciences such as physics (Lin et al., 2004; Gregoryanz et al., 5.5!Portable Double-Sided Laser-Heating System for SMS and Single! Crystal Diffraction Experiments with DACs! ! ! 129! 2005; Dewaele et al., 2008), chemistry and material synthesis (Zerr et al., 2009; Schröder et al., 2011; Palyanov et al., 2011), and especially for geosciences, because they give an opportunity to simulate conditions of the deep Earth’s interior (Mikhaylushkin et al., 2007; Kolesnikov et al., 2009; Wu et al., 2009). The diamond anvil cell technique is a very useful and easy-to-use tool for investigations under high pressure. It was initiated in the late 1950s, and by now has become the most successful method of pressure generation in a multi megabar pressure range (Eremets, 1996; Dubrovinskaia et al., 2010). The laser heating in DACs was first introduced by Ming and Bassett (Ming & Bassett, 1974) and is well described in the literature (Boehler, 2000). There are numerous facilities, including specialized beamlines at the third-generation synchrotrons (Shen et al., 2001; Schultz et al., 2005; Hirose, 2006; Prakapenka et al., 2008), where the DAC laser-heating technique is coupled with different analytical methods for in situ investigations such as the Raman spectroscopy or X-ray diffraction (Boehler et al., 2008; Santoro et al., 2005). However, until recent times all these laser-heating systems were linked to certain equipment or to a beamline. Modern scientific challenges demand higher flexibility in capabilities of research techniques including laser heating in DACs. As a result, since 2009 portable DAC laser-heating systems that can be easily moved between different analytical facilities, including transfer from in-house to a synchrotron or between synchrotron beamlines, began to emerge (Boehler et al., 2009; Dubrovinsky et al., 2009). Still there was no a portable double-sided laser-heating system that would meet all scientific needs. The system described by Boehler et al., (2009) allows double-sided laser heating but requires a rather large space (0.4 m2) for the support. It has never been tested for applications in single crystal studies. Moreover, the design of this system (Boehler et al., 2009) has a drawback – simultaneous temperature and X-ray measurements are not possible because optical components for collecting thermal radiation block and/or shadow the X-ray primary beam or the scattered signal. The system described by Dubrovinsky et al., (2009) was successfully used in routine in house experiments (Gu et al., 2011; Parakhonskiy et al., 2011), the in situ synchrotron X-ray absorption and diffraction investigations (Narygina et al., 2011), and (with some modifications) in synchrotron single-crystal X-ray diffraction experiments 5.5!Portable Double-Sided Laser-Heating System for SMS and Single! Crystal Diffraction Experiments with DACs! ! ! 130! (Dubrovinsky et al., 2010). Its main disadvantage was one-sided laser-heating and, thus, limited applications for only optically thin samples. Here we report the development of a double-sided laser heating system for single-crystal diffraction studies and other experiments, which require movement of the DAC during a data collection. The new system provides with a more homogeneous temperature distribution within the sample (e.g. it allows higher temperatures without possible destructions due to thermal stresses), and permits the use of thicker samples (thus providing an opportunity to use the system in work with materials which are difficult to heat). 5.5.3. Design of the laser-heating system The system consists of two major components – the sources of laser light and two universal laser-heating heads (UniHeads) (Fig. 5.5.1). The UniHeads are based on the finite cutting laser head (Precitec KG) and their functions in the portable laser system are to focus incoming laser beams on the sample within the DAC, to provide with a high magnification imaging of the sample in the DAC with coaxial illumination, and to give access for the multiwavelength spectroradiometry for temperature measurements (Shen et al., 2010). 5.5!Portable Double-Sided Laser-Heating System for SMS and Single! Crystal Diffraction Experiments with DACs! ! ! 131! Fig 5.5.1 Schematic diagram of the optical components of UniHeads. 5.5.3.1. Laser As a laser source we have used two SPI100 modulated fiber lasers with maximum output power of 100 W. The weight of the both lasers is 40 kg and the excitation wavelength is 1071 nm. An additionally integrated red laser source serves as an aid to initial beam targeting. The output of the SPI lasers has a Gaussian power distribution with a diameter of ~3 mm at 1/e2. The output power level can be monitored and controlled by an external analog interface. The fiber lasers can be operated in a continuous or pulse mode with the frequency up to 100 kHz and minimum time of the pulses of less than 10 µs. The lasers require no water-cooling and use only a standard electrical plug. 5.5!Portable Double-Sided Laser-Heating System for SMS and Single! Crystal Diffraction Experiments with DACs! ! ! 132! 5.5.3.2. Focusing optics and beam targeting The output 1071 nm laser light is collected by a π-shaper (MolTech GmbH), 90° bended using a specially coated beam splitter (Beam Splitter 1 in Fig. 5.5.1: the reflectivity above 1050 nm is >99.99%, it is transparent in the 400-900 nm wavelength range), and focused by the UniHead set of lenses with an 80-mm working distance. The original 3 mm in diameter laser beam with the Gaussian intensity distribution is converted into a flat top beam with up to ~50 µm FWHM (Prakapenka et al., 2008). In order to direct the laser beam to the sample and make it coaxial with the incident X-ray beam, we employ carbon mirrors mounted at approximately 45° angle to the UniHeads’ axis (Fig. 5.5.1, 2). The carbon mirrors are made out of polished glassy carbon plates of 1 or 2 mm thickness. They are coated at the optical facility Lab of the University of Bayreuth by a 100 nm thick silver film and, on top of the silver layer, by a 5 nm thick silica film. In case of experiments using the X-ray diffraction, the carbon mirror of the downstream UniHead is mounded under an angle which is slightly below 45° (usually about 41-42 o ) so that and the mirror appears outside of the path of the direct X-ray beam. Thus, we avoid the undesirable diffraction of X-ray on the carbon mirror. The X-ray scattering from glassy carbon of the upstream mirror is blocked by the DAC’s body. 5.5.3.3. Illumination, observation and the spectroradiometry module The illumination of the sample is realized due to the built-in LED in the UniHead (Fig. 5.5.1). The LED is controlled by an external power supply. For observation of the sample in the DAC the high-resolution µEYETM CCD cameras are used. To prevent oversaturation of the cameras by the reflected laser light, KG3 IR filters (SCHOTT GmbH) are exploited. The software of the camera allows enlarging a part of the observed area and tracing a certain position in the image. The UniHead equipped with a modified optical output module for temperature measurements consists of a µEYETM CCD camera, a focusing lens projecting a central part (of about 10 µm in diameter) of the image of the heated spot onto the end of the optical fiber, and a beam-splitter cube (50/50, Edmund Scientific Inc) (Pippinger et al., 2011). The 5.5!Portable Double-Sided Laser-Heating System for SMS and Single! Crystal Diffraction Experiments with DACs! ! ! 133! beam-splitter cube is movable and can be fixed in two positions. If it is fixed outside of the optical path, then all the light radiated by the heated spot transmits into the optical fiber. The cube position within the optical path allows 50% of the light to be transferred to the CCD camera for visual observations. In our experiments we used an Ocean Optics QE65000 spectrometer or Acton SP2300 spectrometer (Princeton Instruments) with the PIXIS400 CCD detector. To prevent the incidence of the laser light onto the detector, 1064 nm notch filters (Edmund Optics Inc.) are used. The temperature measurements are performed by the spectroradiometry method (Shen et al., 2010) i.e. by fitting thermal radiation signals in a given wavelength (usually 600 to 850 µm) range to the Planck radiation function. The software developed by A. Kantor and I. Kantor allows on-line monitoring of the temperature. The system response spectra are calibrated by melting of platinum (Pippinger et al., 2011). 5.5.3.4. System alignment All parts, the UniHeads, a 3D stage for a DAC holder, and the holder with the DAC are mounted on a common aluminum plate (0.35x0.30 m2; the thickness of 0.15 m). In the experiments described below the upstream UniHead was set up horizontally and the downstream UniHead was set up vertically (Fig. 5.5.2). The downstream UniHead is mounted rigidly on the plate and has no degrees of freedom. The sample in the DAC should be aligned with respect to this UniHead using the threedimensional stage. The red alignment laser of the SPI100 can be used to simplify the process. The upstream UniHead is mounted on its own three dimensional stage and should be aligned to the sample within the DAC afterwards. Focusing of the laser beams on the sample is proceeded due to adjustable lenses of the π-shapers (Prakapenka et al., 2008). The last part of the procedure is the alignment of the spectrometers. For this purpose the end of the optical fiber coming to the spectrometer is connected to the visible (in our experiments 532 nm) diode laser whose spot is focused on the sample. By means of the adjustable screws (Fig. 5.5.2c) of the spectroscopic module the spot is placed exactly at the position of the center of the heated area. According to our experience, the mounting of the system and its alignment requires two to three hours. 5.5!Portable Double-Sided Laser-Heating System for SMS and Single! Crystal Diffraction Experiments with DACs! ! ! 134! a) b) 5.5!Portable Double-Sided Laser-Heating System for SMS and Single! Crystal Diffraction Experiments with DACs! ! ! 135! c) Fig. 5.5.2 The double-side portable laser-heating system mounted for experiments at ID09a (a,b) and ID18 beam lines at ESRF (c). 1, holder with DAC; 2, carbon mirrors; 3, focusing optics; 4, π-shaper; 5, beam-splitter cube; 6, CCD camera; 7, LED; 8, three dimensional stages; 9, adjustable screws for spectrometer focusing; 10, MAR555 detector. 5.5.4. Examples of application of the portable laser-heating system 5.5.4.1. Mössbauer spectroscopy investigation of (Mg0.8Fe0.2)O Hyperfine parameters determined from Mössbauer spectra strongly depend on pressure and temperature. Thus the energy domain 57Fe Mössbauer spectroscopy is not only one of the best methods to study iron’s valence and spin state at high pressure, but the probe of choice to test temperature homogeneity in laser-heated DACs. However, conventional energy-resolved Mössbauer spectroscopy utilizes radioactive sources with low brilliance. Therefore high-pressure Mössbauer studies 5.5!Portable Double-Sided Laser-Heating System for SMS and Single! Crystal Diffraction Experiments with DACs! ! ! 136! require too long collection time to be effectively applied for investigations in a laserheated DAC. Recently developing of a Synchrotron Mössbauer Source (SMS) at the Nuclear Resonance beamline (ID 18) (Chapter 5.2) solved this problem. The SMS allows one to perform energy-domain Mössbauer measurements using synchrotron as a source of gamma radiation. In contrast to radioactive sources, the beam emitted by the SMS is nearly fully resonant, has high brilliance and can be focused to a 10x5 µm2 spot size. That opens a possibility for fast and robust measurements in DACs at high pressures and, if coupled with laser heating, at high temperatures. In order to test the double-sided laser heating system with the SMS we selected (Mg0.8Fe0.2)O (ferropericlase, Fp20), since its Mössbauer spectra at high pressure and temperature are already known from our previous work using an externally heated diamond anvil cell and a conventional in-house Mössbauer source (Kantor et al., 2009). Pellets of Fp20 of 30 to 40 µm in diameter with the thickness of about 15 µm were loaded along with a Ne medium (Kurnosov et al., 2008) into a pressure chamber (the initial diameter of 100 µm and the height of about 30 µm) made in a Re gasket. The sample was compressed using a symmetric piston-cylinder type DAC produced at BGI. All measurements were carried out at ID 18 using the SMS, and each spectrum took roughly 10 minutes to collect. The Mössbauer spectrum of the high-spin phase (P < 50 GPa) consists of a single quadrupole doublet arising from octahedrally coordinated Fe2+ (Fig. 5.5.3a) (Kantor et al., 2006). Upon laser heating from only one side (or during double-sided heating when the X-ray beam and the laser spot are not well aligned), the spectrum becomes asymmetric (Fig. 5.5.3b). Since we know from previous experiments on (Mg0.8Fe0.2)O that temperature causes a decrease in quadrupole splitting, we interpret the spectrum in Fig. 5.5.3b as indicating a temperature gradient in the sample between the relatively cold material (grey doublet) and a hotter region (black singlet), corresponding to temperatures of 450 K and 1000 K, respectively. During double-sided heating with the correct alignment of the X-ray beam and laser spot, the Mössbauer spectrum becomes more symmetric (Fig. 5.5.3c) at the temperature of 1300 K. We found that temperature gradients also arise from the double-sided laser heating of thick samples (above about 20 µm), where the interior of the sample remains colder than the surface. 5.5!Portable Double-Sided Laser-Heating System for SMS and Single! Crystal Diffraction Experiments with DACs! ! ! 137! Fig. 5.5.3 Synchrotron Mössbauer source spectra of Fe0.2Mg0.8O at 29 GPa: (a) room temperature; (b) laser heating with misaligned beam; (c) double-sided laser heating with good alignment. The temperatures determined from the centre shift are 450 K (grey doublet in (b)) and 1000 K (black singlet in (b)) and 1300 K (singlet in (c)). The fit residuals are shown below each spectrum. 5.5.4.2. X-ray single crystal diffraction of Fe,Al rich silicate perovskite in double-side laser heated DAC Iron and aluminum bearing magnesium silicate perovskite (Mg,Fe,Si,Al)O3 is likely to be the main component of the Earth lower mantle. Iron and aluminum could significantly affect properties of silicate perovskite, especially due to electronic transitions in Fe2+ and Fe3+. 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