Superiority of nitrate decomposition method for synthesis of K₂NiF₄-type La_xSr_2-xMnO₄ catalysts

K₂NiF₄-type La_0.2Sr_1.8MnO₄ was synthesized by nitrate (ND) and nitrate/acetate (NAD) decomposition methods as well as solid-state reaction. Single-phase oxide was obtained at 550 °C by the ND method just after the decomposition of Sr(NO₃)₂ and at 1000 °C by the NAD method after the decomposition of SrCO₃. The K₂NiF₄-type oxide was hardly formed by the solid-state reaction. In the La-Sr-Mn system, an intermediate compound of SrCO₃, if present or formed during the decomposition process, interfered with the low-temperature formation of the K₂NiF₄-type oxide because of its high decomposition temperature about 1000 °C. The ND method used only metal nitrates and no starting materials with carbon source, so that the low-temperature synthesis of the K₂NiF₄-type oxide was realized without forming obstinate intermediate compound of SrCO₃. The low-temperature synthesis was possible for La_xSr_2-xMnO₄ with the substitution of La (0 < x < 0.5) and not for La_0.2A_1.8MnO₄ (A = Ca and Ba). The effect of A-site cations on the K₂NiF₄-phase formation was discussed from the geometric aspect.