Hexagonal and pyrochlore-type cesium tungstate synthesized from cesium peroxo-polytungstate and their redox chemistry

Thermal decomposition of cesium peroxo-polytungstate, an amorphous precursor, yielded not only pyrochlore-type but also hexagonal WO₃-type cesium tungstate, depending on the Cs/W ratio (x) in the precursors; the former is in the range 0.48 ≤ x ≤ 0.54 and the latter 0.30 ≤ x ≤ 0.34. Mixtures of these two phases were formed in the intermediate region of x. On reduction at 700°C, the pyrochlore phase gave a compound with the same framework, with its cubic cell parameter being elongated from 10.25 to 10.32 Å, but reoxidation resulted in a mixture of pyrochlore and hexagonal phases. In contrast, reduction and oxidation of the hexagonal phase at 600°C was reversible. Powder XRD profile refinements were performed with reduced and oxidized hexagonal cesium tungstates (x = 0.30) in the space group P 6₃/mcm. The former compounds (a = 7.4049(1) and c = 7.6098(1)) are based on an almost idealized h-WO₃ framework with Cs sitting on its large 2b interstices (R_f = 0.0240). The oxidized compounds (7.4012(18) and 7.6728(17)Å R_f = 0.007) possessed a lacunar tungsten sublattice.