Document [original]

Mixed-Metal Monophosphate Tungsten Bronzes Containing

Rhodium and Iridium

Alexander Karbstein,[a] Markus Weber,[a] Dominic Lahr,[a] Jörg Daniels,[a]

Wilfried Assenmacher,[a] Werner Mader,[a] Frank Rosowski,[b] Stephan A. Schunk,[c] and

Robert Glaum*[a]

Solution combustion synthesis followed by annealing in air led

to the MPTB-related phosphates (Rh1/6W5/6O3)8(PO2)4, (Ir1/6W5/

6O3)8(PO2)4(a=5.258(2) Å, b=6.538(3) Å, c=17.322(8) Å), (Rh1/

9W8/9O3)12(PO2)4and (Rh2/21W19/21O3)14(PO2)4. Single-crystals of the

mixed-metal (Rh,W)-MPTBs at m=4 and at m=7 were grown

by chemical vapor transport (CVT). Their crystal structures have

been refined from X-ray single-crystal data {(Rh,W)-MPTB at

m=4:P212121,Z=1, a=5.2232(3) Å, b=6.4966(3) Å, c=

17.3819(9) Å, R1=0.032, wR2=0.075 for 1714 unique reflections,

1524 with Fo>4σ(Fo), 66 variables, 1 constraint, composition

from refinement (Rh0.15W0.85O3)8(PO2)4;(Rh,W)-MPTB at m=7:

P21/n,Z=1, a=5.2510(4) Å, b=6.4949(5) Å, c=26.685(2) Å, β=

90.30(1)°,R1=0.060, wR2=0.163 for 2074 unique reflections,

1894 with Fo>4σ(Fo), 100 variables, comp. from ref.

(Rh0.07W0.93O3)14(PO2)4}. These structure refinements show un-

expected distribution of Rh and W over the available metal

sites. Further characterization (powder reflectance and mag-

netic measurements) of the (Rh,W)-MPTB at m=4 and at m=7

suggest for both phases a homogeneity range with respect to

the Rh/W ratio and the presence of small amounts of W5+

besides Rh3+and W6+. Results of the ligand field analysis for the

reference material Rh(PO3)3, which is containing the octahedral

chromophore [RhIIIO6], are reported (Δo=23200 cm1,B=

490 cm1).

Introduction

Thermodynamically metastable, multinary tungsten phosphates

with ReO3-like XRPD-pattern containing platinum group metals

have recently been identified as catalyst materials for the

formation of maleic anhydride via selective oxidation of n-

butane.[1] Annealing these catalyst materials at temperatures

above 900°C leads to phases with XRPD pattern resembling

those of phosphate tungsten bronzes (PTBs). These mixed-

metal PTBs are expected to contain platinum group metals

(PGM) in well-established chemical environments, which should

lead, following the reasoning of the concepts of single-site

catalysts and site isolation,[2] to even better catalyst perform-

ance than that observed for the disordered solids with ReO3-like

XRPD-pattern. Furthermore, up to now only a few anhydrous

phosphates of rhodium and iridium have been characterized

crystallographically (RhPO4and Rh(PO3)3,[3] Ir(PO3)3[4]). For this

reason further characterization of PGM-containing mixed-metal

MPTBs appeared of additional interest.

Since the discovery of the first tungsten bronze AxWO3(A:

Na, K) with yellow-golden color and metallic luster by Wöhler in

1825,[5] a wide range of bronzes containing transition metals

have been studied.[6] Their structures consist of corner-sharing

MO6octahedra arranged in a 3D-network, which can be

interpreted as a distorted ReO3-type structure. The interstitial

position can be occupied by a large cation Ato build a

perowskite-type structure, which leads to mixed-valency tung-

sten (V,VI) and, as a result, interesting physical properties (e.g.

electrical conductivity, antiferromagnetic ordering or super-

conductivity at low temperatures).[7,8]

Raveau et al. synthesized phosphate tungsten bronzes

(PTB), where ReO3-like slabs of WO6octahedra are separated by

phosphate (PO4) or diphosphate (P2O7) groups. These phos-

phates can be grouped into three classes of PTBs according to

their structural features: the monophosphate tungsten bronzes

with pentagonal tunnels (MPTBp) and the general formula

(WO3)2m(PO2)4with 2�m�14, the monophosphate tungsten

bronzes with hexagonal tunnels (MPTBh)Ax(PO2)4(WO3)2m (A: Na,

K, Pb) with 4�m�10 and the diphosphate tungsten bronzes

(DPTB) Ax(P2O4)2(WO3)2m (A: K, Rb, Ba, Tl) and 4�m�10.[9] Note,

that the A-site is not necessarily required to be occupied by a

cation. It can be empty, thus leading to pure mixed-valent

tungsten phosphates.

[a] A. Karbstein, M. Weber, D. Lahr, Dr. J. Daniels, Dr. W. Assenmacher,

Prof. Dr. W. Mader, Prof. Dr. R. Glaum

Institut für Anorganische Chemie

Rheinische Friedrich-Wilhelms-Universität Bonn

Gerhard-Domagk-Str. 1, D-53121 Bonn (Germany)

E-mail: [email protected]

https://www.glaum.chemie.uni-bonn.de/arbeitsgruppe/mitarbeiter/

glaum

[b] Dr. F. Rosowski

BasCat - UniCat BASF JointLab, Technische Universität Berlin, 10623 Berlin,

Germany

E-mail: [email protected]

https://www.bascat.tu-berlin.de/bascat/menue/about_us/scientific_dir-

ectors/

[c] Dr. S. A. Schunk

hte GmbH, Kurpfalzring 104, 69123 Heidelberg, Germany

E-mail: [email protected]

https://www.hte-company.com/

Supporting information for this article is available on the WWW under

https://doi.org/10.1002/ejic.202100047

Wiley-VCH GmbH. This is an open access article under the terms of the

Creative Commons Attribution Non-Commercial License, which permits use,

distribution and reproduction in any medium, provided the original work is

properly cited and is not used for commercial purposes.

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In particular, the MPTBpphases with their electronic

structure are of interest, because they show phase transitions at

low temperatures, which may lead to superconductivity and

charge localization effects (charge density waves). An inves-

tigation of (WO3)14(PO2)4(m=7) shows two phase transitions at

T1=188 K and T2=60 K. These are related to Peierls distortions

leading to charge density wave states and superconductivity

below T<0.3 K.[8]

Substitution of tungsten(V) in the ReO3-like slabs by other

transition metals is possible. It is already known, that W5+can

be replaced by appropriate combinations of tungsten(VI) with

MIV or MIII according to M IV

1=2WVI

1=2and M III

1=3WVI

2=3, respectively. For

the MPTB (WO3)4(PO2)4(m=2) mixed-metal substitution variants

with M: VIII, IV, CrIII, FeIII, MoIII, IV and TiIV have been reported.[10] In

addition to the described MPTBs at m=2, for M: Sc, V, Cr, Fe,

Mo, Ru, Rh, In and Ir the ortho-pyrophosphates MIII-

(WO2)2(P2O7)(PO4) of identical sum formula (with respect to the

MPTBs) but different structure were obtained.[10]

In this paper we report on synthesis, thermal behavior and

characterization of several mixed-metal (Rh,W)- and (Ir,W)-

MPTBs. For characterization results from XRPD, SXRD, electron

microscopy, UV/vis-spectroscopy, and magnetic measurements

are presented.

Results and Discussion

Synthesis

The new mixed-metal (mm) MPTBs (Rh1/6W5/6O3)8(PO2)4, (Rh1/9W8/

9O3)12(PO2)4, (Rh2/21W19/21O3)14(PO2)4and (Ir1/6W5/6O3)8(PO2)4have

been obtained as single-phase, micro-crystalline powders

(XRPD) by solution combustion synthesis (SCS)[11] followed by

annealing of the combustion products in air. For SCS Rh(NO3)3

(Umicore, Hanau) or Ir(acac)3(Umicore, Hanau), (NH4)6W12O39-

(H2O)4.8 (Alfa Aesar, Karlsruhe), H3PO4(VWR, Darmstadt), and the

polydentate ligand glycine (Labochem International, Heidel-

berg) were dissolved in water (molar ratio metal : glycine=1:3).

Conc. HNO3(Fisher Scientific, Schwerte) in excess was added as

oxidant to the solution to prevent the reduction of rhodium or

iridium during the combustion. To our observation, formation

of the PGM metals is irreversible under the described reaction

conditions and has to be avoided. The solutions were

evaporated at temperatures #�100°C and thereafter ignited in

a preheated furnace at #=400°C, where a self-propagating

combustion took place and yielded black, voluminous, amor-

phous (XRPD) powders. These intermediates were ground and

annealed in air with stepwise rising of the temperature up to

1000°C for (Rh1/6W5/6O3)8(PO2)4and (Ir1/6W5/6O3)8(PO2)4and up to

1100°C for (Rh1/9W8/9O3)12(PO2)4and (Rh2/21W19/21O3)14(PO2)4. Thus

obtained powders display a dark-green color for the rhodium

and black for the iridium compounds. XRPD pattern were

recorded after each annealing step to control the progress of

product formation until the thermodynamically stable mm-

MPTB phase was obtained single-phase (see XRPD pattern in

Figure 1 and Figures S1-S4). In case of (Rh1/9W8/9O3)12(PO2)4the

described procedure led always to (Rh2/21W19/21O3)14(PO2)4as

minor by-phase. We attribute this outcome to the close

proximity of chemical composition of the two compounds and

the rather small amounts of material used for synthesis, due to

the high cost of rhodium.

Experiments aiming at “(Rh2/9W7/9O3)6(PO2)4” (m=3), “(Rh2/

15W13/15O3)10(PO2)4“ (m=5) or “(Rh1/12W11/12O3)16(PO2)4” (m=8) led

to the neighboring MPTB phases. In case of iridium only (Ir1/6W5/

6O3)8(PO2)4was formed. The mixed-metal MPTBs (M1/3W2/

3O3)4(PO2)4(m=2) with M=Rh, Ir were never observed. Instead,

formation of M(WO2)2(P2O7)(PO4)2(M=Rh, Ir) did occur, as

previously described by Roy et al.[12]

Lattice parameters of the MPTBs were determined using the

program SOS[13] and α-SiO2(Merck, Darmstadt) as internal

standard (see Table 1). Interestingly, the lattice parameters a

and bwhich are parallel to the ReO3-type slabs of the (Rh,W)-

MPTBs are smaller compared to the pure tungsten MPTBs,

whereas the parameter c(perpendicular to the ReO3-type slabs)

is enlarged (Table 1).

Figure 1. XRPD pattern (IP Guinier technique, Cu-Kα1) of (Rh1/6W5/6O3)8(PO2)4

(top) and (Rh2/21W19/21O3)14(PO2)4(bottom) with simulations based on SXRD

data. (Ir1/6W5/6O3)8(PO2)4and (Rh1/9W8/9O3)12(PO2)4(middle) with a simulation

based on (Rh2/21W19/21O3)14(PO2)4(grey) and (WO3)8(PO2)4[14] or (WO3)12(PO2)4[15]

(black) with an even distribution of rhodium/iridium over all three tungsten

sites.

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Progress of phase formation and thermal decomposition

For (Rh2/21W19/21O3)14(PO2)4, the (Rh,W)-MPTB at m=7, as

representative example, the progress of phase formation

starting from the amorphous intermediate obtained by SCS is

summarized in Figure 2. It shows, rather unexpectedly, strong

kinetic effects during phase formation. After SCS (400°C ignition

temperature; hot spot temperature up to 900°C for a few

seconds) the intermediate exhibits a diffraction pattern typical

of an amorphous solid. Heating to 750°C leads to less

“amorphous background” and development of rather broad

Bragg peaks indicating the formation of a thermodynamically

metastable phase with a ReO3-related pattern. The chemical

composition of this phase, (Rh2/21W19/21O3)14(PO2)4, can be re-

written as (RhIII2/27WVI19/27PV6/27)(O25/9&2/9) to emphasize its rela-

tion to ReO3and to show the oxygen-deficiency as a

consequence of the average oxidation state of the cations. This

and related ReO3-like phases are obtained with high reproduci-

bility by the described protocol and they show remarkable

thermal stability against transformation into the thermodynami-

cally stable equilibrium phases. However, without SCS and

initial annealing temperatures too high these phases were

never obtained.

Surprisingly, transformation (equilibration) of the single-

phase homogenous ReO3-like phase to the microcrystalline

powder of (Rh2/21W19/21O3)14(PO2)4(m=7) does not occur

directly, but via the apparently kinetically favorable formation

of the (Rh,W)-MPTB at m=4 (depleted in WO3with respect to

the starting composition) and a WO3-rich phase with ReO3-

related XRPD pattern. Thus, as can be taken from Figure 2,

above 800°C a pattern similar to (WO3)8(PO2)4(MPTB at m=4)

begins to emerge from the amorphous background. Simulta-

neously, the lattice parameter aof the ReO3-like phase shrinks

slightly from 3.79 Å to 3.71 Å (see Table 2) and is thus

approaching the value given in literature for cubic WO3.[17] We

are therefore relating this change to a depletion of the ReO3-

like phase of rhodium and phosphorus and an enrichment by

WO3. The powder obtained after annealing at 1000°C (Figure 2)

clearly shows, according to its XRPD pattern, an off-equilibrium

mixture of three phases. The presence of the WO3-rich, ReO3-

like phase (~16%wt) and of the intermediate (Rh,W)-MPTB (m=

4; ~47%wt), which has a higher content of rhodium and

phosphorus than the starting mixture is apparently kinetically

controlled. The equilibrium phase (Rh2/21W19/21O3)14(PO2)4(m=7)

is already present in the mixture at ~37%wt. Eventually, after

annealing at 1100°C the targeted (Rh,W)-MPTB at m=7 is

obtained as single-phase powder. Further heating up to 1200°C

of this (Rh,W)-MPTB in air leads to decomposition to P4O10

vapor, WO3(monoclinic after quenching to ambient temper-

Table 1. Lattice parameters from XRPD data of (Rh,W)- and (Ir,W)-MPTBs in comparison to the pure tungsten phosphates.

Compound a[Å] b[Å] c[Å] β[°]

m=4 (orthorhombic)

(WO3)8(PO2)4[14] 5.285(2) 6.569(1) 17.351(3)

(Rh1/6W5/6O3)8(PO2)45.223(1) 6.513(2) 17.356(6)

(Rh0.15W0.85O3)8(PO2)4[a] 5.2232(3) 6.4966(3) 17.3819(9)

(Ir1/6W5/6O3)8(PO2)45.258(2) 6.538(3) 17.332(8)

m=6 (orthorhombic)

(WO3)12(PO2)4[15][a] 5.2927(7) 6.5604(7) 23.549(3)

(Rh1/9W8/9O3)12(PO2)45.229(2) 6.540(3) 23.599(7)

m=7 (monoclinic)

(WO3)14(PO2)4[16][a] 5.291(1) 6.557(2) 26.654(8) 90.19

(Rh2/21W19/21O3)14(PO2)45.245(2) 6.507(2) 26.693(8) 90.10(4)

(Rh0.07W0.93O3)14(PO2)4[a] 5.2510(4) 6.4949(5) 26.6854(19) 90.30(1)

[a] Lattice parameters from single-crystal measurement at room temperature (crystals from CVT, composition from structure refinement).

Figure 2. XRPD pattern (IP Guinier technique, Cu-Kα1) of (Rh2/21W19/

21O3)14(PO2)4after several consecutive steps of annealing starting with the

combustion product from SCS, rel. amounts (wt %) estimated with

MATCH!.[19]

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ature), and rhodium (see Eq. 1). The latter two solid phases

were identified by XRPD.

The progress of phase formation observed for the other

mm-MPTBs under consideration, (Rh1/6W5/6O3)8(PO2)4, (Ir1/6W5/

6O3)8(PO2)4and (Rh1/9W8/9O3)12(PO2)4is very similar to the one

described for (Rh2/21W19/21O3)14(PO2)4(see Tables S5-S7). How-

ever, there is variation in the temperature required to obtain

the single-phase mm-MPTB and in their decomposition temper-

atures. Single-phase powders of (Rh1/6W5/6O3)8(PO2)4and (Ir1/6W5/

6O3)8(PO2)4(both m=4) are formed already at about 800°C,

(Rh1/9W8/9O3)12(PO2)4(m=6) around 1050°C and (Rh2/21W19/

21O3)14(PO2)4(m=7) around 1100°C. Short annealing periods of

a few hours only slightly above their decomposition temper-

atures leads to stepwise loss of P4O10 and the formation of

elementary rhodium and the (Rh,W)-MPTB with the next higher

m(higher WO3content). Overall, the decomposition temper-

ature of the (Rh,W)-MPTBs increases with decreasing content of

rhodium/P4O10. This observation fits to the decomposition

behavior of pure rhodium phosphates (Rh(PO3)3, RhPO4), where

the activity of P4O10 is higher and decomposition in air begins

at 950°C.[18]

(1)

Based on their thermal decomposition temperatures and

the assumption that p(O2) should exceed 0.2 bar for a

detectable decomposition effect, an estimate of the thermody-

namic data DfH0

298;S0

298;Cp(coefficients A, B, C) of (Rh1/6W5/

6O3)8(PO2)4and (Rh2/21W19/21O3)14(PO2)4was possible using the

data of the binary oxides Rh2O3, WO3and P4O10. The estimation

follows a procedure applied by Schäfer for LaPO4.[20] For a

summary of thermodynamic data used in the context of this

paper, see Table S1.

Crystallization by chemical vapor transport

Crystals of the (Rh,W)-MPTBs (Rh1/6W5/6O3)8(PO2)4(m=4), (Rh1/

9W8/9O3)12(PO2)4(m=6) and (Rh2/21W19/21O3)14(PO2)4(m=7) have

been grown by chemical vapor transport (CVT)[21] in sealed silica

tubes (l=14 cm, d=1.5 cm) with chlorine as transport agent.

These show a dark greenish color, have edge lengths up to

300 μm and are isometric with a rectangular shape (Figure 3).

Pre-synthesized, single-phase powders (from SCS followed

by annealing in air) were used as starting materials for CVT.

Small amounts of PtCl2(~15 mg) were added for in situ

generation of chlorine, the transport agent. Temperature

gradients 950!900°C for (Rh1/6W5/6O3)8(PO2)4and 1090!

1050°C for (Rh1/9W8/9O3)12(PO2)4and (Rh2/21W19/21O3)14(PO2)4were

applied. Ampoules were kept in the furnaces for 14 d. Under

the given experimental conditions no decomposition of the

(Rh,W)-MPTBs was observed. However, transport of (Rh1/9W8/

9O3)12(PO2)4and (Rh2/21W19/21O3)14(PO2)4in the temperature

gradient 950!900°C led to deposition of (Rh1/6W5/6O3)8(PO2)4in

the sink due to non-stationary behavior of the system. For a

general explanation of non-stationary transport behavior

see.[21,22]

Chemical vapor transport of (Rh1/6W5/6O3)8(PO2)4with chlor-

ine as transport agent was modelled with the computer

program CVTrans[23] using thermodynamic data given in

Table S1. According to the modelling and in agreement with

literature on CVT of WO3and Rh2O3the transport active gas

species are WO2Cl2, RhCl3, P4O10, and O2(see Eq. 2).[18,21]

(2)

Crystal structure analyses

The (Rh,W)-MPTB at m=4 and m=7 are isotypic to their pure

tungsten MPTB counterparts (WO3)8(PO2)4(P212121,Z=1)[14] and

(WO3)14(PO2)4(P21/n,Z=1).[16] Table 3 provides a summary on

data collection and refinement of the crystal structures. Atomic

coordinates and isotropic displacement parameters are shown

in Table 4 and Table 5 (Anisotropic displacement parameters

see Table S2 and Table S3). Refinements for both (Rh,W)-MPTB

were hampered by twinning, cation disorder, and possibly

stacking faults all of which are typical for monophosphate

tungsten bronzes as has been pointed out in literature several

times.[25] Due to the limited quality of the refinements we refrain

from discussing interatomic distances (see Figures S8 and S9) in

detail. Nevertheless, the distances are very similar to those

observed for the pure MPTBs. The refinements do provide clear

information on the metal sites with rhodium/tungsten mixed

Table 2. Lattice parameter a[Å] of the metastable intermediate phases with ReO3-like XRPD pattern as a function of temperature [°C] during formation of

the different (Rh,W)-MPTBs.

600 700 750 800 1000

(Rh1/6W5/6O3)8(PO2)4not det. 3.786(8) 3.783(7) not det. not det.

(Rh1/9W8/9O3)12(PO2)43.792(9) 3.785(7) not det. 3.776(3) 3.713(7)

(Rh2/21W19/21O3)14(PO2)43.780(5) 3.781(6) not det. 3.776(4) 3.718(7)

Figure 3. Crystals of (Rh1/6W5/6O3)8(PO2)4(left, middle) and (Rh2/21W19/21

O3)14(PO2)4(right) obtained by CVT.

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occupancy. In addition, the refinements for both (Rh,W)-MPTB

suggest slightly higher tungsten content than expected for the

ideal compositions with rhodium(III) and tungsten(VI):

(Rh0.15W0.85O3)8(PO2)4instead of (RhIII1/6WVI5/6O3)8(PO2)4(m=4)

and (Rh0.07W0.93O3)12(PO2)4instead of (RhIII2/21 WVI19/21O3)14(PO2)4

(m=7); Table 3, Table 4, and Table 5. Details on the refinements

are given in the Experimental Section. Graphical representations

of the two crystal structures are given in Figure 4. Both show

ReO3-like slabs, which are four octahedra wide for the m=4

phase and seven for the phase at m=7. These slabs are linked

by phosphate tetrahedra. In both structures the [MO6] octahe-

dra joining the ReO3-like slabs together are linked to three [PO4]

tetrahedra and they are exclusively occupied by tungsten.

These are sites M2 in the m=4 phase and M3 in the m=7

phase (see Table 4 and Table 5; numbering of atom sites as in

the original papers on the pure tungsten MPTBs). In contrast,

[MO6] with only one or no adjacent phosphate group are partly

occupied by rhodium. For the m=7 phase even a systematic

increase of Rh3+occupancy with increasing distance from the

phosphate tetrahedra (from the interface between the ReO3-like

slabs) is observed. This cation ordering is rather surprising since

valence sum considerations would suggest just the opposite

ordering scheme for Rh3+/W6+, as is shown by the following

considerations along the lines of Paulings rules and Browns

equal valence rule.[27] In the crystal structures of the mono-

phosphate tungsten bronzes all oxygen atoms are twofold

coordinated (P+W, P+Rh, W+W, W+Rh, Rh+Rh). Assuming

ideal valence distribution within the coordination polyhedra

[PO4] (vP-O =5/4), [WO6] (vW-O =6/6), and [RhO6] (vRh-O =3/6) leads

to “overbonded” oxygen when coordinated by P+W (ΣvO=9/

4). For W+W (ΣvO=12/6), P+Rh (ΣvO=7/4), W+Rh (ΣvO=9/6),

Rh+Rh (ΣvO=6/6) the valence sum for oxygen drops from ideal

to strongly “underbonded”. Allowing slight alteration of

interatomic distances would result in some relaxation.[28]

However, exchanging tungsten by rhodium in [MO6] octahedra

linked to phosphate would already avoid all unfavorable

valence sums for oxygen, i.e. those in contrast to Paulings rule

of preservation of local electroneutrality.[27] Yet, this is not

observed. One explanation might lay in an additional contribu-

tion to lattice energy from charge density modulation along the

crystallographic c-axis in these structures. In other words, slabs

with over bonded and under bonded oxygen (surplus of

positive and negative charges, respectively) would stack along

the crystallographic c-axis.

All crystals of (Rh1/9W8/9O3)12(PO2)4(m=6) selected for SXRD

structure analysis showed diffuse streaks in sections of the

reciprocal space (see Figure S10). No reasonable integration of

the reflections was possible. We relate this problem to the

presence of severe stacking faults and variable thickness of the

ReO3-like slabs.

Table 3. Summary on data collection and refinement of the crystal

structures of (Rh0.15W0.85O3)8(PO2)4and (Rh0.07W0.93O3)14(PO2)4.

Empirical formula Rh1.33W6.67P4O32 Rh1.33W12.67P4O50

Structural formula (Rh0.167W0.833O3)8(PO2)4(Rh0.095W0.905O3)14(PO2)4

Refined formula (Rh0.146W0.854O3)8(PO2)4Rh1.04W12.96P4O32

Formula weight 2012.40 3413.47

Crystal system orthorhombic monoclinic

Space group P212121(no. 19) P21/n(no. 14)

T(K); λ(Å) 293(2); 0.71073 293(2); 0.71073

a(Å)[a] 5.2232(3) 5.2510(4)

b(Å)[a] 6.4966(3) 6.4949(5)

c(Å)[a] 17.3819(9) 26.6854(19)

β(°)[a] 90 90.30(1)

Z1 1

V (Å3); Dcalc (gcm3) 589.82(5); 5.665 910.09(12); 6.228

Crystal dimensions

(mm3); color

0.055×0.03×0.03;

dark green

0.068×0.042×0.021;

dark green

F(0 0 0) 874 1466

Absorption semi-empirical

(multiscan)[24]

semi-empirical

(multiscan)[24]

μ(mm1); ext. coef. 33.648; 0.0026(2) 40.453; 0

Measured refls. ind. 10119; 1714 7312; 2074

No. of parameters 58 100

Theta range (°) 3.35–29.91 3.05–27.59

Index ranges 7�h�7, 9�k�9,

24�l�23 6�h�6, 7�k�8,

34�l�33

GooF 1.108 1.131

Rint 0.0653 0.1217

R1[I>2σ(I)] 0.0315 0.0600

R1; wR2(all data) 0.0374; 0.0742 0.0649; 0.1687

[a] Lattice parameters from single-crystal measurement.

Table 4. (Rh1/6W5/6O3)8(PO2)4. Atomic coordinates and isotropic displace-

ment parameters.

Atom x y z s. o. f. Ueq [Å2]

W1 0.2502(2) 0.41042(6) 0.04236(2) 0.708(6) 0.0142(2)

Rh1 0.2502(2) 0.41042(6) 0.04236(2) 0.292(6) 0.0142(2)

W2 0.25049(15) 0.24727(5) 0.37161(2) 1 0.00929(12)

P 0.2478(10) 0.0604(3) 0.18620(11) 1 0.0079(4)

O1 0.963(2) 0.2898(14) 0.0006(6) 1 0.018(2)

O2 0.213(3) 0.2505(13) 0.1383(5) 1 0.026(2)

O3 0.228(2) 0.1205(10) 0.2701(4) 1 0.016(2)

O4 0.033(2) 0.0487(15) 0.4064(5) 1 0.017(2)

O5 0.532(2) 0.1202(15) 0.4044(5) 1 0.018(2)

O6 0.549(2) 0.399(2) 0.3281(6) 1 0.025(2)

O7 0.022(2) 0.477(2) 0.3288(6) 1 0.022(2)

O8 0.281(2) 0.4091(12) 0.4599(4) 1 0.021(2)

Table 5. (Rh2/21W19/21O3)14(PO2)4. Atomic coordinates and isotropic displace-

ment parameters.

Atom x y z s. o. f. Ueq [Å2]

W1 0 0.5 0 0.822(16) 0.0222(5)

Rh1 0 0.5 0 0.178(16) 0.0222(5)

W2 0.0011(4) 0.16144(13) 0.11289(3) 0.954(12) 0.0146(4)

Rh2 0.0011(4) 0.16144(13) 0.11289(3) 0.046(12) 0.0146(4)

W3 0.0001(3) 0.00523(11) 0.32958(2) 1 0.0088(3)

W4 0.0020(5) 0.32824(14) 0.44491(3) 0.875(13) 0.0200(4)

Rh4 0.0020(5) 0.32824(14) 0.44491(3) 0.125(13) 0.0200(4)

P1 0.506(2) 0.3156(7) 0.29117(17) 1 0.0084(9)

O1 0.553(4) 0.018(6) 0.0033(18) 0.5 0.010(7)

O2 0.540(4) 0.158(2) 0.5602(6) 1 0.019(4)

O3 0.039(4) 0.167(2) 0.3847(6) 1 0.021(4)

O4 0.038(4) 0.012(3) 0.1772(6) 1 0.024(4)

O5 0.485(6) 0.381(2) 0.2375(6) 1 0.023(3)

O6 0.206(5) 0.133(4) 0.4713(9) 1 0.039(6)

O7 0.709(4) 0.212(4) 0.4725(8) 1 0.030(5)

O8 0.301(4) 0.043(3) 0.0871(7) 1 0.019(4)

O9 0.795(4) -0.028(3) 0.0892(8) 1 0.025(4)

O10 0.286(4) 0.312(3) 0.1476(8) 1 0.025(5)

O11 0.780(4) 0.364(3) 0.1491(7) 1 0.018(4)

O12 0.229(3) 0.227(3) 0.3022(7) 1 0.018(4)

O13 0.694(4) 0.157(3) 0.2979(8) 1 0.021(4)

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Electron microscopic investigation

SAED pattern and HRTEM images taken from crushed crystals of

(Rh0.15W0.85O3)8(PO2)4and (Rh0.07W0.93O3)14(PO2)4are displayed in

Figure 5. The lattice parameters derived from both SAED

patterns accord well with the values obtained from XRD. No

hints were found for the presence of superstructures.

The HRTEM images are in agreement with the structure

models deduced from SXRD refinement, as it can be seen from

overlaying a structural image based on coordination polyhedra

(Figure 5). The characteristic fish-bone pattern with rows of

ReO3-like slabs and phosphate groups in between are nicely

matched. A prominent characteristic of these MPTB structures

are the pentagonal tunnels, which are formed between the

ReO3-slabs and phosphate groups and display a better white-

contrast every second time along [001]. In previous electron-

microscopic studies this effect was also observed and inves-

tigated. The contrast in MPTB’s is very sensitive to crystal tilting

near [100] and the crystal thickness.[29] The varying contrast

therefore can be explained with a slight misalignment to the

zone axis [100] and an unfavorable crystal size.

Images taken with lower magnifications show highly

ordered ReO3-like slabs with equal thickness, therefore, provid-

ing no hints on stacking faults. This result disagrees with the

observation, that the intensities of the reflections (0 0 2) and (0

0 4) (see Tables S4–S7 and Figures S1–S4) calculated from SXRD

data differ from the observed intensities of the XRPD data. It is

also in contrast to the observation of significant diffuse

scattering observed by SXRD for all (Rh,W)-MPTB. Several

explanations are possible: Stacking faults occur in some crystals

only and are not seen in the areas investigated by HRTEM or

the ratio of rhodium/tungsten differs between ReO3-like slabs

within a crystal.

Electronic spectra and magnetic behavior of the (Rh,W)-MPTB

Powders of the (Rh,W)-MPTBs (m=4 and 7) are dark green in

contrast to the orange-yellow of Rh(PO3)3(C-type; [RhIIIO6]

chromophore), which we discuss here as reference material. The

color difference is in line with powder reflectance measure-

ments of the three compounds shown in Figure 6. For Rh(PO3)3

the two strong absorption bands at 21500 cm1and

28000 cm1can be assigned to the ligand field transitions

1A1!1T1g and 1A1g!1T2g of the octahedral [RhIIIO6] chromophore.

Graphical evaluation according to Tanabe and Sugano[30] results

in B=430 cm1and Δ=23200 cm1. These values are in good

agreement with those determined for the [RhIIIF6] chromophore

in K3RhF6(B=460 cm1,Δ=22300 cm1).[31] In contrast to

Rh(PO3)3and K3RhF6the (Rh,W)-MPTBs show a broad absorption

band in the NIR/vis ranging from 5000 cm1to 14000 cm1and

a second in the vis/UV ranging from 19000 cm1to 29000 cm1

(see Figure 6). The minimum around 17000 cm1readily

explains their green color. The absorption in the NIR/vis is

reminiscent to those observed for blue heteropolytungstates,[32]

mixed-valent tungsten oxides, e.g. WO3-δ,[33] W20O58[34] or for

electrochromic tungsten bronzes, e.g. NaxWO3[35] and might

thus point to the presence of some W5+besides W6+and Rh3+.

The ligand-field transitions for the [RhIIIO6] chromophore are not

resolved in the (Rh,W)-MPTBs, most likely they are super-

imposed by LMCT (O2!W6+).

Angular overlap modelling (AOM)[36] has been used to

reproduce the d-electron energy levels of Rh3+in Rh(PO3)3. For

the AOM calculations the PC program CAMMAG[37] in a modified

version[38] was used. Best fit AOM parameters obtained from the

optical spectrum of Rh(PO3)3are B=490 cm1(β=0.80), C=

1960 cm1,ζ=1408 cm1,eσ(RhO) ~ d(RhO)5.0 and

eσ(RhO)max =11300 cm1. Isotropic π-interaction RhO was

assumed to be 1/4 of the corresponding σ-interaction

{eπ(RhO)=1/4 eσ(RhO}. As can be seen from the comparison

in Figure 6, results from AOM match well with the observed

energies for the transitions 1A1g!3T1g,1A1g!3T2g,1A1g!1T1g, and

1A1g!1T2g of the [RhIIIO6] chromophore.

For RhIII(PO3)3and the (Rh,W)-MPTBs with the ideal compo-

sitions (RhIII1/6WVI5/6O3)8(PO2)4and (RhIII2/21WVI19/21O3)14(PO2)4one

could expect diamagnetic behavior due to the d0configuration

of W6+and the 1A1g electronic ground state of Rh3+(d6,low-

spin). However, considering a second-order Zeeman effect for

Figure 4. Crystal structures of (Rh1/6W5/6O3)8(PO2)4(left) and (Rh2/21W19/21O3)14(PO2)4(right) with schematic coordination polyhedra. Projections along [100]; dark

grey: phosphate tetrahedra; grey: WO6octahedra; light grey: octahedra MO6with mixed occupancy Rh/W (graphics software DIAMOND v.4.5.1.[26])

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Rh3+would lead to a small magnetic moment independent of

temperature (TIP) according to van Vleck’s equation.[39] Indeed,

this expectation is perfectly met by the temperature independ-

ent susceptibility observed for Rh(PO3)3,χTIP = +

100·106emu·mol1(Figure 7). This value is also in agreement

with measurements for Rh3+(e.g. RhCl3:χTIP = +85·106

emu·mol1;[40] Rh2(SO4)3·12 H2O: χTIP = +95·106emu·mol1;[40]

[Rh3O(CH3COO)6(py)3]ClO4:χTIP = +280·106emu·mol1;[41] all χ

per formula unit). Furthermore, the results from angular overlap

modelling that already matched the excited state energies for

the [RhIIIO6] chromophore are fully consistent with the observed

TIP for rhodium(III) metaphosphate (Figure 7). In contrast, the

(Rh,W)-MPTBs show much higher molar susceptibilities

(χTIP(Rh1.164W6.836P4O32)= +299·106emu·mol1,

χTIP(Rh1.04W12.96P4O32)= +470·106emu·mol1;both obtained

from a linear fit to the respective graphs of χmol ·Tvs. Tshown in

Figure S11, plot of μeff vs. Tsee Figure S12) which cannot be

rationalized on basis of the exclusive presence of [RhIIIO6]

(Figure 7).

Figure 5. Top: Selected area electron diffraction (SAED) pattern of (Rh1/6W5/6O3)8(PO2)4(a) and (Rh2/21W19/21O3)14(PO2)4(c) in [1 0 0] zone axis orientation. Bottom:

Fourier-filtered HRTEM image (CM30 T) of (Rh1/6W5/6O3)8(PO2)4(b) and (Rh2/21W19/21O3)14(PO2)4(d) in [1 0 0] zone axis orientation with overlaid structure of the

MPTBs.

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Subtracting χTIP(Rh3+)= +100·106emu·mol1from χmol, exp

of the (Rh,W)-MPTBs still leaves for both significant contribu-

tions to the overall magnetic susceptibility. Two explanations,

both related to the presence of W5+(d1electronic system, 2T2g

electronic ground state for the octahedral chromophore [W5+

O6]), appear to be possible. Neither would lead to simple Curie-

Weiss-behavior. Magnetic behavior of sodium tungsten bronzes

NaxWO3might serve as comparison.[42] Therein reported

susceptibilities, due to the Pauli paramagnetism[43] of delocal-

ized electrons, are far too small to provide a reasonable

explanation for the TIP of the (Rh,W)-MPTBs. A second approach

for understanding might be based on the paramagnetism of

localized electrons in W5+. Yet, due to strong spin-orbit

coupling and the wide variability of ligand field effects in 5d

ions predictions on the paramagnetic behavior of W5+contain-

ing compounds are hard to make. Comparison to simple,

magnetically well characterized systems (e.g. CsWVF6[44],

CsWVCl6[45]) suggests a rather high content of up to 50% W5+of

the total tungsten content of the (Rh,W)-MPTBs. This seems to

be unrealistically high by any means. Given the overall rather

small susceptibilities of the (Rh,W)-MPTBs leaves eventually just

the assumption of contributions of unidentified paramagnetic

impurities as explanation. In this context it is worth noting that

magnetic susceptibility data obtained for two batches of the

(Rh,W)-MPTB at m=4 (powder synthesis and crushed crystals

from CVT) were almost identical. Clearly, characterization of the

electronic structure of the (Rh,W)-MPTBs requires additional

work, which is ongoing.

Chemical composition of the (Rh,W)-MPTBs

Seemingly, different experimental data (synthesis, SXRD, optical

spectroscopy, magnetic measurements) for the (Rh,W)-MPTBs

suggest different chemical composition for these phases. Thus,

a critical review of these data with respect to the chemical

composition of the phases under investigation is justified.

Syntheses based on the ideal compositions (exclusively Rh3+

and W6+) (Rh1/6W5/6O3)8(PO2)4(phase at m=4) and (Rh2/21W19/

21O3)14(PO2)4(phase at m=7) led to single phase powder

samples according to XRPD analysis of the solid reaction

products. By-phases as consequence of deviations in composi-

tion will only be observable for an estimated amount higher

than 3%wt.

Structure refinements from SXRD data suggest small

deviations from the ideal Rh/W ratio with slightly higher

tungsten content than expected. Even though there is some

correlation between site occupancy factors for Rh/W and the

corresponding displacement parameters, we believe that the

rhodium content of the two (Rh,W)-MPTBs is indeed slightly

lower than expected for the ideal composition. Yet, the

composition from SXRD might be regarded as the lower limit of

rhodium content. While no hints on stacking faults in the

(Rh,W)-MPTBs were obtained from HRTEM and SAED, diffuse

scattering observed in SXRD patterns point to such defects

which possibly will lead to deviations in stoichiometry.

Color and powder reflectance spectra of the (Rh,W)-MPTBs

are strikingly different than what is observed for the perfectly

stoichiometric and electronically well-defined metaphosphate

RhIII(PO3)3. Strong bands in the NIR/vis region suggest mixed-

valency W5+/W6+, yet already less than 1% of W5+would be

sufficient to cause such an absorption behavior, as is evidenced

by the deep blue color of under-stoichiometric WO3-δ.[33,34]

Magnetic susceptibility data of (Rh1/6W5/6O3)8(PO2)4and (Rh2/

21W19/21O3)14(PO2)4do show, as expected, weak paramagnetism.

Figure 6. Powder reflectance spectra of (Rh1/6 W5/6O3)8(PO2)4(top) and

(Rh2/21W19/21O3)14(PO2)4(middle) diluted with BaSO4(50:50 ratio) in compar-

ison to a spectrum of pure Rh(PO3)3C-type (bottom). Ticks at the bottom are

representing zero-phonon excitations for Rh(PO3)3from the best fit angular

overlap modelling (AOM; see text).

Figure 7. Molar (per formula unit) magnetic susceptibilities of

Rh1.164W6.836P4O32 (m=4), Rh1.04W12.96P4O32 (m=7) and Rh(PO3)3in a field of

104G.

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Yet, the absolute numbers cannot be rationalized solely based

on the TIP expected for Rh3+.

Results from EDS analyses (see Table 6) of crystals from CVT

seem also to indicate a slightly smaller ratio Rh/W than related

to the ideal compositions of the (Rh,W)-MPTBs. Yet, these

analyses (including a ZAF correction) overestimate the light

elements (phosphorus content) and underestimate heavy

elements (Rh, W).

Summarizing these critical considerations, we believe that

the obtained (Rh,W)-MPTBs do indeed show a slightly lower

ratio Rh/W than that of the “ideal” compositions. Clearly, these

phases do contain a small but significant amount of W5+.

Structural investigations did not give any hint on variations of

the metal/phosphorus ratio in the (Rh,W)-MPTBs.

Conclusions

In extension of previous work it is shown that in MPTBs

(WO3)2m(PO2)4substitution of W5+by appropriate combinations

of Rh3+(or Ir3+) and W6+is possible. Thus, new mm-MPTBs with

the ideal compositions (Rh1/6W5/6O3)8(PO2)4, (Rh1/9W8/9O3)12(PO2)4,

(Rh2/21W19/21O3)14(PO2)4(m=4, 6, 7) and (Ir1/6W5/6O3)8(PO2)4(only

m=4) were obtained.

Solution combustion synthesis with subsequent annealing

of the combustion products provides access to these new mm-

MPTBs. Furthermore, the progress of phase formation during

annealing (amorphous!ReO3-related phase!two-phase off-

equilibrium mixtures!mm-MPTB equilibrium phase) allows a

unique glimpse into the processes related to stepwise crystal-

lization and equilibration of an amorphous, solid starting

material. Thermodynamically metastable mixed-metal phos-

phates with ReO3-related XRPD pattern were established as

single-phase intermediates. Their formation emphasizes the

hitherto not recognized versatility of SCS for synthesis of

thermodynamically metastable solids.

Thermal decomposition behavior of the (Rh,W)-MPTBs is in

agreement with only a rather small stabilization of these phases

relative to W2O3(PO4)2, WO3, and Rh2O3and the thermal

decomposition products thereof WO3, Rh and gaseous P4O10.

Therefore, crystallization of the (Rh,W)-MPTBs by chemical vapor

transport (transport agent chlorine) is possible.

Structure refinement of the (Rh,W)-MPTBs at m=4 and 7

show cation distributions Rh/W that are in contrast to Paulings

electroneutrality rule. An overall stabilization due to alternating

layers with small positive/negative charges could be a reason.

Optical spectra and magnetic behavior of the obtained

(Rh,W)-MPTBs suggest mixed-valency W5+/6+and, as a conse-

quence, a small tungsten surplus with respect to the ideal

formulae. Eventually, in the course of our investigation on the

(Rh,W)-MPTBs a ligand field analysis for C-type Rh(PO3)3based

on powder reflectance and magnetic measurements has been

carried out as reference for the chromophore [RhIIIO6].

Experimental Section

X-ray powder diffraction.Powder diffraction pattern for phase

identification and purity control were recorded at ambient

temperature using an imaging plate (IP) Guinier camera (HUBER

G670, Cu-Kα1radiation, λ=1.54059 Å, 15 minutes exposure time

in the angular range 4°�2θ<100°). The observed powder

diffraction pattern of (Rh1/6W5/6O3)8(PO2)4, (Rh2/21W19/21O3)14(PO2)4,

(Rh1/9W8/9O3)12(PO2)4and (Ir1/6W5/6O3)8(PO2)4are compared to

simulations which are based on the data from single crystal

structure analysis or isotypic compounds with matched lattice

parameters and a suitable chemical composition (Figure 1).

Tables containing the assigned reflections from the Guinier

photograph are available online as Supplementary Material

(Table S1–4).

Single-crystal X-ray diffraction.Suitable crystals of (Rh1/6 W5/6

O3)8(PO2)4and (Rh2/21W19/21O3)14(PO2)4were selected carefully

under a polarizing microscope and glued to glass fibers.

Diffraction data (graphite-monochromated Mo-Kα1 radiation; λ=

0.71073 Å) were collected using a Bruker Nonius k-CCD area-

detector diffractometer (BRUKER AXS B.V., Delft, Netherlands)

which was controlled by the Nonius “Collect” software.[46] For

data processing the software package “HKL2000: Denzo &

Scalepack” was employed.[47] A semi-empirical absorption correc-

tion based on multiscans was applied to the diffraction data

using the program package Platon.[48]

(Rh1/6W5/6O3)8(PO2)4: Starting parameters for the structure refine-

ment were obtained by the dual space structure solution algorithm

(SHELXT[49]) and refined with a full-matrix least-squares technique

(SHELX-97,[50] WinGX[51]). Space group P212121as for the pure

tungsten MPTB (WO3)8(PO2)4[14] was assumed. The two independent

metal sites were in a first step treated as fully occupied by tungsten.

Site W1 (notation as given in literature[14] for (WO3)8(PO2)4) showed

in contrast to W2 a rather large displacement parameter. In the

next refinement step for site M1 mixed occupancy W/Rh was

assumed. Refinement was carried out using the EXYZ and EADP

constraints for W/Rh on site M1 and assuming full occupancy. No

hint was found for mixed occupation of site M2. The refinement led

to 1.17 Rh and 6.83 W in the unit cell instead of 1.33 Rh and 6.67 W,

as it was expected from charge neutrality (Rh3+, W6+) consider-

ations. It should be noted in this context that any effort to constrain

the ADPs for sites M1 and M2 yielded significantly higher rhodium

contents in the unit cell (e.g. 1.75 Rh; 6.25 W) but drastically worse

residuals. Eventually, anisotropic displacement parameters could be

introduced for sites M1 and M2. Phosphorus and oxygen could only

be refined isotropically, due to occurrence of physically mean-

ingless (“n.p.d.”) atomic displacement parameters. Racemic twin-

ning was accounted for and led to the volume ratio V1/V2=0.52(5).

The remaining highest electron density (1.82 e/Å3) was found close

to O8 with similar residual electron density close to other oxygen

sites. We attribute these maxima to the limitations of the isotropic

refinement. The deepest minimum in electron density (2.78 e/Å3)

was found close to site M1. It is interpreted as an effect of

discontinuation of the Fourier series. The flaws in this refinement

Table 6. Ideal and analytical composition of “(Rh1/6W5/6O3)8(PO2)4” (m=4)

and “(Rh2/21W19/21O3)14(PO2)4” (m=7).

m=4m=7

Rh [%] W [%] P [%] Rh [%] W [%] P [%]

ideal composition 11.11 55.56 33.33 7.41 70.37 22.22

SXRD 9.7(1) 57.0(1) 33.33 5.8(5) 72.(5) 22.22

EDS (REM)[a] 11.0(8) 50(2) 39(3) 7.1(6) 67(3) 26(2)

[a] Area measurement of single crystals

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(despite acceptable residuals R1=3.2%, wR2=7.4%) mirror those

reported for many other refinements of MPTB type compounds.

(Rh2/21W19/21O3)14(PO2)4: Starting parameters for the structure refine-

ment were obtained by Direct Methods (SHELX-97[50]). Space group

P21/nas for the isotypic MPTB (WO3)14(PO2)4[16] as assumed in the

refinement (full-matrix least-squares refinement: SHELX-97,[50]

WinGX[51]). Assuming full occupancy of the metal sites, three (W1,

W2, W4) out of four tungsten sites showed unusually high isotropic

displacement parameters. Therefore, except for site W3, mixed

occupancy Rh/W was introduced using the EXYZ and EADP

constraints for each of the three sites. Even after accounting for

twinning (mirror plane perpendicular to the crystallographic c-axis,

as reported in literature for (WO3)14(PO2)4[25]) refinement of aniso-

tropic displacement parameters was only possible for the metal

and phosphorus sites. Eventually, for oxygen atom O1 occupying

the Wyckhoff site 2da split position with half occupancy of the

general site 4ewas introduced. From the refinement 1.04 rhodium

and 12.96 tungsten in the unit cell were obtained in contrast to

1.33 Rh and 12.67 W for the ideal composition. The value of Rint =

0.12 indicates low quality of the diffraction data, which is typical for

many refinements of MPTB structures.

Electron microscopic investigation.TEM images were conducted on

a FEI-Philips CM30 T/LaB6microscope operated at 300 kV and

equipped with a Gatan CCD for image recording. The samples were

prepared by suspending a little amount in cyclohexane using an

ultrasonic bath, applying a drop of the dispersion on a holey

carbon film reinforced by a copper grid and evaporating the

solvent. Digital TEM images were processed using the Gatan Digital

Micrograph software.[52]

To get an image along the [100] zone axis, a suitable crystal was

tilted along dynamic electron diffraction (“Kikuchi-lines”) towards a

simulated pattern. A diffraction pattern was taken and evaluated to

confirm the zone axis, afterwards a HRTEM image was prepared.

The HRTEM images were Fourier filtered using the Gatan Micro-

scopy Suite. After an analysis of the real space distances, a model

from the SXRD solution could be superimposed over the HRTEM

image. EDS analysis were carried out on a Hitachi SU3800 with a

tungsten cathode operated at 10 kV. For quatification the EDAX

detector “Octane Elect Super EDS” (peltier element cooling) and a

ZAF correction were used. Results were obtained as mean values

from several examined single-crystals.

Electronic absorption spectroscopy.Powder reflectance spectra

were measured at room temperature using modified CARY-14 (UV-

region) and CARY-17 (vis/NIR-region) spectrophotometers (OLIS,

USA) equipped with integrating (Ulbricht) spheres. Four different

setups for measurement (step width, band width, scan rate,

detector) were applied to cover the range from 5000 cm1to

40000 cm1. In the UV range (200 nm–600 nm) 800 data points

(step width 0.5 nm, scan rate 1 nm · sec1, band width 0.6 nm), in

the visible range (300 nm–900 nm) 600 data points (step width

1.0 nm, scan rate 1 nm · sec1, band width 1.2 nm) and in the near-

infrared range (600 nm–2200 nm) 500 data points (step width

4 nm, scan rate 2 nm · sec1, band·width 5 nm) were recorded. For

detection in the UV/vis region a photomultiplier detector (PMT) was

used, in case of the vis region with an aperture width of 0.06 nm. In

the near-infrared region a semiconducting lead(II)sulfide detector

with variant gap widths (1.4 nm–2.2 nm) was utilized. The spectra

are represented as a ration of K/S vs. wavenumber (Kubelka-Munk

function).[53]

Magnetic measurements.Magnetic measurements were carried out

using a vibrating sample magnetometer (Quantum design, USA).

The strength of the external magnet field was 104Oe in the

temperature range from 2 K to 300 K. Magnetic susceptibilities

were measured during cooling and heating. At each temperature

step a series of ten measurements were taken. Only close to

ambient temperature, slight differences between the data obtained

on cooling or heating were observed. The susceptibility data shown

in Figure 7 have been corrected for capsule and diamagnetic

contributions.[54] Furthermore, a low content of ferromagnetic

impurity was corrected with a Honda-Owen plot. Therefore,

measurements with a different external field (0 to 20000 Oe) were

conducted at 150 and 300 K for all samples. All susceptibilities are

given for 1 mole (formula unit) of the compound. In case of the

MPTBs the sum formula according to SXRD was used.

Deposition Numbers 2062902 (for Rh0.15W0.85O3)8(PO2)4(phase at

m=4)), and 2062903 (for Rh0.07W0.93O3)12(PO2)4(phase at m=7))

contain the supplementary crystallographic data for this paper.

These data are provided free of charge by the joint Cambridge

Crystallographic Data Centre and Fachinformationszentrum Karls-

ruhe Access Structures service www.ccdc.cam.ac.uk/structures.

Acknowledgements

We thank Norbert Wagner for magnetic measurements, Nils

Kannengießer for measurements of the UV/vis/NIR spectra and

Volker Bendisch (all of Inorganic Chemistry at Bonn University) for

photographs of the single-crystals. Funding by BASF SE, coopera-

tive work and discussions throughout the cooperation project

“PGM Phosphates” are gratefully acknowledged. Open access

funding enabled and organized by Projekt DEAL.

Conflict of Interest

The authors declare no conflict of interest.

Keywords: Equilibrium relations ·MPTB ·Rhodium ·Iridium ·

Solid-state chemistry ·Solid-state structure

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Manuscript received: January 19, 2021

Revised manuscript received: March 10, 2021

Accepted manuscript online: March 12, 2021

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