CuX_2O_6 and Ba_3CuX_2O_9 (X = Nb, Ta): influence of the preparation conditions on phase formation and phase composition

摘要

The title compounds were prepared by conventional solid state reaction and by thermal decomposition of freeze-dried oxalate precursors using different temperature regimes. The thermal decomposition of an appropriately composed oxalate precursor enables the direct synthesis of pure monoclinic CuNb_2O_6, thermodynamically stable only at θ ≤ 700℃. The solid state reaction leads to the orthorhombic columbite type phase or to a mixture of the orthorhombic and monoclinic phases. A new metastable CuTa_2O_6-trirutile phase was synthesized by thermal decomposition of an oxalate precursor. Above 800℃, the decomposition results in the already known perovskite-like structure. Tetragonal perovskites Ba_3Cu_(1+x)Ta_(2-x)O_(9-1.5x) and Ba_3Cu_(1+x)Na_(2-x)O_(9-1.5x) were synthesized by solid state reaction and by precursor decomposition. The phase formation and homogeneity of the perovskites strongly depends on the reaction conditions.