K2NiF4-type La0.2Sr1.8MnO4 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 LaxSr2-xMnO4 with the substitution of La (0 < x < 0.5) and not for La0.2A1.8MnO4 (A = Ca and Ba). The effect of A-site cations on the K2NiF4-phase formation was discussed from the geometric aspect.
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