Hexagonal and Pyrochlore-Type Cesium Tungstate Synthesized from Cesium Peroxo-Polytungstate and Their Redox Chemistry

Abstract

Thermal decomposition of cesium peroxo-polytungstate, an amorphous precursor, yielded not only pyrochlore-type but also hexagonal WO 3-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 6 3/ mcm. The former compounds ( a = 7.4049(1) and c = 7.6098(1)) are based on an almost idealized h-WO 3 framework with Cs sitting on its large 2 b 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.