Abstract

K_1.33P₄W₈O₃₂ belongs to the homologous series of monophosphate tungsten bronzes with hexagonal tunnels A_x(PO₂)₄(WO₃)_2m with 0 < x < 2 and 4 < m < 14. K_1.33P₄W₈O₃₂ exhibits a phase transition at T_c = 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 × b × 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) Å, b = 5.3282(1) Å, c = 8.926(4) Å, and β = 100.65°. The structure was refined within the superspace approach with superspace group P2₁(α, 0, γ) with α = 0.5 and γ = 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 Å 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.