Crystal structure of monophosphate tungsten bronze K₁.₃₃P₄W₈O₃₂ at 110 K

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Dusek, Michal ; Lüdecke, Jens ; van Smaalen, Sander:
Crystal structure of monophosphate tungsten bronze K₁.₃₃P₄W₈O₃₂ at 110 K.
In: Journal of Materials Chemistry. Bd. 12 (2002) Heft 5 . - S. 1408-1414.
ISSN 1364-5501
DOI: https://doi.org/10.1039/B109982P

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Abstract

K1.33P4W8O32 belongs to the homologous series of monophosphate tungsten bronzes with hexagonal tunnels Ax(PO2)4(WO3)2m with 0 < x < 2 and 4 < m < 14. K1.33P4W8O32 exhibits a phase transition at Tc = 165 K, that is characterized by an anomaly in the temperature dependence of the electrical resistivity and by the occurrence of extra reflections in the X-ray scattering. Here we report the 2a times b times c superstructure at T = 120 K{,} as it was measured by X-ray diffraction with synchrotron radiation. The superstructure is found to be monoclinic P21 with lattice parameters a = 13.373(7) A{,} b = 5.3282(1) A{,} c = 8.926(4) A{,} and small beta = 100.65degree. The structure was refined within the superspace approach with superspace group P21(small alpha{,} 0{,} gamma) with small alpha = 0.5 and gamma = 0. Final agreement was obtained at R = 0.032 (R = 0.025 for the main reflections and R = 0.153 for the superstructure reflections). The maximum shift out of the average position was 0.25 A for one of the oxygen atoms. An analysis of the displacements and variations of interatomic distances is used to show that the mechanism of the phase transition is to resolve the internal strain between the K atoms and the surrounding PO4 groups. The comparison with the CDW structure of (PO2)4(WO3)2m shows that the phase transition is not a charge-density wave{,} but rather that the anomaly in the resistivity is caused by the changes in the structure when going through the phase transition.