CUX2O6 AND BA3CUX2O9 (X = NB, TA) - INFLUENCE OF THE PREPARATION CONDITIONS ON PHASE-FORMATION AND PHASE-COMPOSITION

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 approp riately composed oxalate precursor enables the direct synthesis of pur e monoclinic CuNb2O6, thermodynamically stable only at theta less than or equal to 700 degrees C, The solid state reaction leads to the orth orhombic columbite type phase or to a mixture of the orthorhombic and monoclinic phases. A new metastable CuTa2O6-trirutile phase was synthe sized by thermal decomposition of an oxalate precursor. Above 800 degr ees C, the decomposition results in the already known perovskite-like structure. Tetragonal perovskites Ba3Cu1+xNb2-xO9-1.5x and Ba3Cu1+xTa2 -xO9-1.5x were synthesized by solid state reaction and by precursor de composition. The phase formation and homogeneity of the perovskites st rongly depends on the reaction conditions.