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-delta-O48) 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 4e 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(x)W1-x/6O3 (x = 0.3) in the space group P6(3)22. The result with R(F) = 6.31% indicated that its framework (W6-delta-O-18 with delta = 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 KxW1-x/6O3) was investigated in a KPF6/propylene carbonate solution at room temperature. The potential of KxWO3 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.