Evidence for a supercooled plastic-crystal phase in solid ethanol

E idence o a supe cooled plas ic-c ys al phase in solid e hanol A. S ini asan and F. J. Be mejo Ins i u o de Es uc u a de la Ma e ia, Consejo Supe io de In es igaciones Cien ı ´ icas, Se ano 123, E-28006 Mad id, Spain A. de And e ´s Ins i u o de Ciencia de Ma e iales de Mad id, Sede B, Consejo Supe io de In es igaciones Cien ı ´ icas, Uni e sidad Au o ´noma de Mad id, Facul ad de Ciencias, C-IV, Can oblanco, E-28049 Mad id, Spain J. Dawidowski Ins i u o de Es uc u a de la Ma e ia, Consejo Supe io de In es igaciones Cien ı ´ icas, Se ano 123, E-28006 Mad id, Spain J. Zu ´n ˜iga Depa amen o de Fı ´sica de la Ma e ia Condensada, Uni e sidad del Pais Vasco, Euskal He iko Uni e si a ea, Facul ad de Ciencias, E-48960 Bilbao, Spain A. C iado Depa amen o de Fı ´sica de la Ma e ia Condensada, Uni e sidad de Se illa, and Ins i u o de Ciencia de Ma e iales, Consejo Supe io de In es igaciones Cien ı ´ icas, Facul ad de Ciencias, P.O. Box 1065, E-41080 Se illa, Spain ~Recei ed 18 Oc obe 1995; e ised manusc ip ecei ed 8 Decembe 1995! The exis ence o an o ien a ionally diso de ed cubic phase o solid e hanol is e ealed by x- ay di ac ion and Raman spec oscopic measu emen s. Such a phase, whose exis ence was pos ula ed some ime ago on he basis o speci ic-hea measu emen s, is p oduced by quenching below some 95 K a plas ic c ys al o med upon mel ing and subsequen annealing o he opologically diso de ed ~glassy!solid. The ele ance o he p esen indings o cu en discussions on glassy dynamics is analyzed in some de ail. Despi e ecen ad ances in ou unde s anding o he mi- c oscopic mechanisms d i ing he dynamics o glasses a in e media e ~abo e 1 K! empe a u es,1mos s udies ega d- ing opologically diso de ed solids ~i.e., eal glassy ma e i- als!s ill encoun e subs an ial di icul ies when ying o go beyond desc ip ions o he da a by means o phenomenologi- cal app oaches. This con as s wi h a mo e de eloped unde - s anding ega ding he dynamics o diso de ed c ys als o med by ei he mixed2o chemically homogeneous ~plas ic and glassy c ys als3,4!ma e ials, which in common wi h s uc u al glasses exhibi some ea u es conside ed as inge - p in s o he glassy dynamics. Such de elopmen s a e, how- e e , es ic ed o ma e ials which being diso de ed in some quan i y ~chemical composi ion, ela i e molecula o ien a- ions, e c.!s ill ha e an unde lying c ys al la ice, hus en- abling he calcula ion o a numbe o physical p ope ies ~i.e., coupling be ween molecula ansla ions and o a ions, so ening o elas ic cons an s due o diso de , anomalous di- elec ic beha io , e c.!in e ms o well-es ablished echniques.2The ele ance o such indings o he unde - s anding o eal glasses was no explo ed in de ail, pe haps due o he ac ha mos o he exis ing ma e ials can ei he be con e ed in o opological o diso de ed-c ys al s a es, ex- cep ions o such a ule being a e ~amo phous and hexagonal ice being an example which in ol es a he di e en ou es o p epa a ion which pose se e e condi ions when ying o compa e he wo in e ms o ansi ion empe a u es!. The exis ence o some polymo phic phase in solid e hanol which could be p epa ed unde mild he mal condi ions was known o exis since he wo k o Haida e al.5who obse ed calo ime ic anomalies dis inc om hose associa ed wi h he glass ansi ion (Tg597 K!, he mel ing o he s able c ys al (Tm5169 K!, o c ys alliza ion o he supe cooled liquid in o such a phase (Tc '125 K!. In ac , he p esence o a well-de ined exo he mic anomaly a some 115 K was in e p e ed as he onse o a c ys al phase ~c ys al II!which once g own exhibi ed a calo ime ic ansi ion in o ano he c ys alline phase ~glassy c ys al!, showing small di e ences in hea capaci y wi h hose o he glass ob ained by apid cooling o he no mal liquid, bu e idencing a calo ime ic en opy be ween ha o he glass and he s able c ys al. A NMR elaxa ion measu emen epo ed la e on6showed ha he dynamics o he eal glass and ha o he glassy c ys al as sampled by he alues o he second momen o he NMR sha ed some analogies, whe eas inspec ion o he ela i e alues o he elaxa ion imes e ealed some sys ema ic di - e ences be ween bo h. Ou aim he e is o es ablish he na u e o he c ys al II and glassy c ys al phases o his ma e ial. I , as expec ed, a su- pe cooled plas ic c ys al ~SPC!can be o med by eezing he o a ional eedom o he plas ic c ys al ~PC!, i would hen p o ide an excellen benchma k ma e ial whe e quan i- a i e s udies on he ole o di e en deg ees o eedom ~posi ional and o ien a ional!on he dynamics o he glass a in e media e empe a u es could be ca ied in de ail. Spec oscopic g ade e hanol was used wi hou u he pu- i ica ion, a oiding exposu e o a mosphe e o p e en cap- u e o wa e apou . The x- ay di ac ion measu emen s we e pe o med using a S oe di ac ome e , using he Cu K a line l51.5418 Å, being he samples con ained in glass PHYSICAL REVIEW B 1 APRIL 1996-IVOLUME 53, NUMBER 13 53 0163-1829/96/53~13!/8172~4!/$10.00 8172 © 1996 The Ame ican Physical Socie y capilla ies cooled by an Ox o d Ins umen s c yos eam coole . The Raman spec a we e eco ded on an X-YDilo spec ome e equipped wi h an enhanced pho odiode a ay. Sli wid hs gi ing nominal esolu ions o 1.8 and 3.7 cm21 we e employed o c ys al and glass spec a, espec i ely. Exci a ion was achie ed by an A 1sou ce lasing a 514 nm and deli e ing some 10 mW o he sample which was con- ained wi hin 5 mm qua z ubes. These we e kep wi hin a Ox o d Ins umen s lux c yos a used o keep empe a u es be ween 10 K and ambien . The sca e ed ligh was collec ed on he S okes side a igh angles. Sample p epa a ion ega ding he no mal glass and s able ~monoclinic!c ys al phases is s aigh o wa d since i only in ol es a quench below Tg597 K a a a e no slowe han 6 K min21~glass!and a subsequen annealing a some 130 K o a leas a hal hou ~c ys al!. As ega ds he p epa a ion o he supe cooled-plas ic-c ys al phase, ou own expe ience indica es ha ep oducible esul s a e ob ained i he supe - cooled liquid o med upon mel ing he no mal glass is al- lowed o anneal o some minu es a T5100 K ollowed by a apid quench below he Ta597 K ansi ion. A mac o- scopic phase sepa a ion occu s du ing annealing whe e a anslucen solid phase seg ega es om he clea liquid. Figu e 1 shows a plo compa ing he x- ay sca e ed in- ensi ies o he h ee phases o solid e hanol. The monoclinic c ys al spec um is ully compa ible wi h he c ys al s uc u e desc ibed in Re . 7 which co esponds o a uni cell con ain- ing ou molecules, wo o which a e independen in he asymme ic uni o Pc symme y. The e e ed wo molecules ha e a di e en in e nal s uc u e ~con o ma ion!, showing dihed al CuCuOuH angles o some 297° ~gauche!and 179° ~ ans!wi h also signi ican a ia ion o he bond-angle alues.7The c ys al s uc u e shows in ini ely bonded zigzag chains joined by OuH•••O bonds o leng h 2.716 Å. In ull con as , he glass shows a b oad spec um wi h a main di - ac ion peak cen e ed a Q'1.65 Å21and a wid h o '0.5 Å21. The pa ame e s o such a peak a e close o hose ound o a chemically ela ed glass ~me hanol!discussed in some de ail in he pas ,8and which we e ound o ep esen 9 a sys em o long, winding chains, wi h a co ela ion leng h @de ined by he ex en in eal space whe e he g( ) in e mo- lecula dis ibu ion unc ion shows no iceable oscilla ions#o some 15 Å. The exis ence o a high-symme y c ys al phase is demons a ed by he di ac og am o Fig. 1 ~middle!, which shows ha mos o he in ensi y a ises om wo B agg e lec ions a Q51.651 Å21~in ense!and Q52.335 Å21 ~weak!, alongside wi h small aces o con amina ion by minu e amoun s o he monoclinic phase and a b oad, di use backg ound. Such a di ac ion pa e n can be unde s ood as a ising om a body-cen e ed-cubic s uc u e wi h la ice con- s an a55.381 Å, he ~110!and ~200!, espec i ely, being he obse ed e lec ions. Assuming wo molecules pe uni cell, he assignmen would co espond o a solid wi h a mac o- scopic densi y o 982.5 kg m23, close o he alue a Tgas ex apola ed om abula ed alues o he no mal supe - cooled liquids10 ( 5958.2 kg m23). The alue o he la - ice cons an men ioned abo e becomes e y close o ha o 5.377~4!Å co esponding o he adimension o he mono- clinic cell.7On he o he hand, he close loca ion o he ~110! e lec ion wi h he wa e ec o cha ac e is ic o he peak maximum in he glass phase p o ides a clea example o he physical meaning o he i s di ac ion peak in glasses, an issue which was he ocus o some ecen con o e sies.11 In ac , compa ison o di ac og ams o glass and cubic c ys al clea ly sugges s ha only he wid h o such a peak should bea a clea physical signi icance in a glass. Because o he small numbe o obse ed B agg e lec- ions, u he de ails ega ding he s uc u e o such a cubic phase could only be de i ed om addi ional independen measu emen s. Be ween hose, a compa ison o he Raman spec a o he solid wi hin i s h ee ~cubic, monoclinic, and glass!phases was chosen and he mos ele an esul s a e displayed in Fig. 2. The immedia e consequence o such a compa ison ega ds he exis ence o a s ong s a ic diso de in he cubic phase as demons a ed by he absence o well- esol ed phonon peaks below some 40 meV. Tha such a diso de is o s a ic ~o ien a ional!na u e is u he a i ied by he absence o any he mal anomaly o his phase below ou ac ual T580 K, which should be p esen i subs an ial o a- ional mo ions we e s ill p esen . I seems hen clea ha he cubic phase ep esen s a SPC o glassy c ys al phase, whe e he la ge o a ional mo ions p esen abo e 97 K ha e ozen in o a s ongly o ien a ionally diso de ed phase. In ac , spec- a measu ed du ing annealing ime show in addi ion o he peaks shown in he SPC phase an addi ional s ong di use backg ound. The amoun and kind o diso de ~i.e., s a is ical o h ough some p e e en ial di ec ion!canno be in e ed om he a ailable da a, al hough he simili ude be ween he FIG. 1. X- ay di ac ion pa e ns o ~a! he posi ionally diso - de ed glass a 80 K, ~b! he supe cooled cubic c ys al phase a 80 K, and ~c! he s able ~monoclinic!c ys al a 120 K. The small peak a Q'1.786 Å21in ~b!comes om a small con amina ion o he monoclinic c ys al phase. 53 8173BRIEF REPORTS low- equency Raman spec a ~below some 20 meV!and he coincidence o he posi ion o he in ense B agg e lec ion wi h he maximum o S(Q) in he canonical glass pinpoin s he exis ence o ema kable co ela ions be ween molecula o ien a ions as i is known o occu in he glass phase o o he alcohols.9,12 The Raman spec a o he glass and SPC solid show a he simila shapes o he equency dis ibu ions. Because o he iso opy o he glass and he o ien a ional diso de in he SPC, he Raman in ensi ies could be conside ed o be di- ec ly ela ed o he ib a ional densi y o s a es h ough he usual exp ession ZR( )}I( ) /@n( )11#, whe e in he absence o addi ional in o ma ion a quad a ic equency de- pendence o he C( ) ligh - o- ib a ion coupling unc ion is assumed ~i.e., comple e andomness o couplings!. No ice om he inse in Fig. 2 ha he main di e ence be ween canonical and SPC spec a ega ds he posi ion o he maxima which in he SPC phase is shi ed some 1 meV abo e ha o he canonical glass, as well as a a he small excess o modes below 6 meV o he glass wi h espec o he SPC. An assessmen o he physical soundness o such esul s is p o ided by e alua ing he ha monic con ibu ion o he cons an - olume hea capaci y om he ZR( ) es ima e o he ib a ional densi ies o s a es desc ibed abo e. To a oid p oblems ega ding he no maliza ion o ZR( ) o absolu e uni s only he a ios C g/C SPC o hea capaci ies o he glass and SPC a e conside ed. Such a quan i y yields a alue o 1.34 o be compa ed wi h ha o 1.60 de i ed om da a gi en in Re . 13. On he o he hand, he maxima o C (T)/T3cu es appea in he SPC phase shi ed some 1.8 K abo e ha o he s uc u al glass, a ac also bo ne ou by calo ime ic measu emen s whe e he maxima o he SPC sample is loca ed a some 6.5 K whe eas ha o he glass appea s a '4.5 K. In consequence, and wi h he abo e p o isos in mind, i seems clea ha he cubic SPC phase o e hanol he e s udied co esponds o he same one explo ed by calo ime ic means.5,13 This being he case, he di e ence in speci ic hea s a Tgdiscussed by Joha i and o he s14 upon mel ing he glass and ha a ising om he SPC-plas ic c ys al ansi ion can be a ionalized on a mo e quan i a i e basis. In ac , compa ison o he jumps in Cpa he glass ansi ion and o a ional a es poin s Tg5Ta597Ko 35and23JK 21mol21e idences ha some 66% o he jump upon mel ing he glass can be accoun ed by he onse o apid molecula o a ions. Such a pe cen age becomes e en la ge ~75%! o he jump in en- opy, and goes o some 63% o he con igu a ional en opy. In conclusion, he exis ence o a o ien a ionally diso - de ed phase in a sys em conside ed since many decades ago as one o he clea es examples o a glass- o ming liquid has been demons a ed. F om he p esen da a as well as om ha al eady a ailable, he SPC solid can be igh ly consid- e ed as a phase wi h an in e media e kind o diso de be- ween he g ound s a e ~monoclinic c ys al!and he glass. In his espec , i s de ailed s udy will open up he possibili y o sepa a ing he e ec s o posi ional and o ien a ional diso de on di e en magni udes usually conside ed in s udies on glassy dynamics. This wo k was suppo ed in pa by DGICYT G an No. PB92-0114-C04. 1Fo a ecen compendium o discussions on his opic, see P o- ceedings o he II In e na ional Discussion Mee ing on Relax- a ion in Complex Sys ems, edi ed by K.L. Ngai e al. @J. Non- C ys . Solids 172-174,~1994!#. 2R.M. Lynden-Bell and K.H. Michel, Re . Mod. Phys. 66, 721 ~1994!. 3The Plas ic C ys alline S a e, edi ed by J.N. She wood ~Wiley, New Yo k, 1979!. 4A. Loidl and R. Bo ¨hme , in Diso de E ec s on Relaxa ional P ocesses, edi ed by R. Riche and A. Blumen ~Sp inge -Ve lag, Be lin, 1994!, p. 659. 5O. Haida, H. Suga, and S. Seki, J. Chem. The modyn. 9, 1133 FIG. 2. Raman equency dis ibu ions ZR( ) 5I( ) /@n( )11#a 10 K o ~a! he glass phase, uppe ame ~b! he SPC phase mid ame, and ~c! he monoclinic c ys al. The inse in ~b! ep esen s an enla gemen o he equency dis ibu ions o he glass and SPC phases. In ensi ies a e gi en in a bi a y uni s. 8174 53 BRIEF REPORTS ~1977!. Also M. Hugo, K. Schlu ¨ e , and A. Wu ¨ linge , Z. 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Vazquez e al. ~Wo ld Sci- en i ic, Singapo e, 1995!, p. 212. Also, M.A. Ramos, Q.W. Zhow, J.A. Mo eno, and S. Viei a, in P oceedings o he Wo k- shop on Non-equilib ium Phenomena in Supe cooled Fluids, Glasses and Amo phous Ma e ials, Pisa, I aly, 1995 ~Wo ld Sci- en i ic, Singapo e, in p ess!. 14G.P. Joha i, Philos. Mag. B 41,41~1980!; G.P. Joha i and J. Pe ez, Mol. Phys. 83, 235 ~1994!. 53 8175BRIEF REPORTS