Thermal decomposition of cesium peroxo-polytungstate, an amorphous precursor, yielded not only pyrochlore-type but also hexagonal (\mathrm{WO}{y}) type cesium tungstate, depending on the (\mathrm{Cs} / \mathrm{W}) ratio ((x)) in the precursors; the former is in the range (0.48 \leq x \leq 0.54) and the latter (0.30 \leq x \leq 0.34). Mixtures of these two phases were formed in the intermediate region of (x). On reduction at (700^{\circ} \mathrm{C}), the pyrochlore phase gave a compound with the same framework, with its cubic cell parameler being elongated from 10.25 to (10.32 \AA). but reoxidation resulted in a mixture of pyrochlore and hexagonal phases. In contrast, reduction and oxidation of the hexagonal phase at (600^{\circ} \mathrm{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} / \mathrm{mcm}). The former compounds ((a=7.4049(1)) and (c=7.6098(1))) are based on an atmost ideatized (\mathrm{h}-\mathrm{W}){4}) framewerk with C's silting on its large (2 b) interstices (\left(R{\mathrm{F}}=\mathbf{0 . 0 2 4 0 )}\right.). The oxidized compounds (\left(7.4012(18)\right.) and (7.6728(17) \AA . R_{\mathrm{f}}=\mathbf{0 . 0 0 7 )}) possessed a lacunar tungsten sublattice. 9) 1943
Acadenic Press. Ine.