Synthesis and Electrochemical Property of Framework Structure Oxides Derived from Peroxo-Polytungstates

Peroxo-polytungstate anion, formed by the direct raction of metallic tungsten powder with hydrogen peroxide, was precipitated as its salts with relatively large cations such as K⁺, Rb⁺, Cs⁺, Tl⁺, Ba²⁺, Pb²⁺ and NH₄⁺. These precursor salts decomposed at relatively low temperature in air gave oxides with the framework structures based on six-membered rings of corner-sharing WO₆ coordination octahedra. One of frameworks was a defective pyrochlore type skeleton (W₁₈_₃O₄₈) derived from Cs- and Tl-salt, in which the cations were accommodated at its large 8 d interstices. Another was basically a hexagonal WO₃ type skeleton given by the rest, in which cations except NH₄⁺ occupied the half of 4 e positions in its one dimensional tunnels. The compound from NH₄-salt was hexagonal WO₃ itself though its c-axis was significantly shorter than the reported value. Powder XRD profile refinement was performed for K_xW1-_X/₆O_B (X=0. 3) in the space group P6_a22. The result with R_F=6. 31% indicated that its framework (W₆_₅O_i8 with δ= 0.3) took an almost idealized hexagonal WO₃ arrangement in terms of geometry, though it was defective in terms of composition. Electrochemical intercalation of K⁺ iono the empty hexagonal WO₃ framework synthesized from ammonium salt (of which cell dimensions were almost identical to those of Kx W_1-x/6O₃) was investigated in a KPF₆/propylene carbonate solution at room temperature. The potential of K_xWO₃ vs. Li (initially about 3.0 V) showed a sharp drop at x=0.35, indicating K ions were intercalated into tunnels of the framework, because this value agreed with the maximum accommodation of K in the tunnel sites. XRD recorded before and after intercalation of K showed that the reaction was perfectly topotactic. Moreover, the change of cell dimenstions accompanied by the intercalation was very small.