Oxides of the AMO3 and A2MO4-type: structural stability, electrical conductivity and thermal expansion

The structural and chemical stabilities, electrical conductivity, and thermal expansion of the A2−aAa′MO4−x oxides (A=La; A′=Sr; M=Mn, Fe, Co, Ni) with the perovskite-related K2NiF4-type structure were investigated and compared with the characteristics of perovskite-type oxides AMO3−x containing the same cations. The K2NiF4-type manganites, ferrites, cobaltites and nickelates are assumed to be reduction products of the corresponding perovskite-type oxides. The thermodynamic stabilities, in terms of reversible oxygen desorption, were higher than those of the corresponding perovskite-type oxides. Within the range of oxygen partial pressure (pO2) from air to argon/H2/H2O, the oxidation states of the M cations were determined. The comparison of the oxidation states of M in AMO3−x and (AMO3−x)·AO gives evidence on the stabilizing influence of the AO interlayer on the perovskite layer. ectrical conductivity of the A2MO4 oxides was of p-type and reached values close to 100 S cm−1 at high oxygen partial pressures and 800 °C for nickelates and cobaltites. The thermal expansion of K2NiF4-type oxides is generally lower than that of the comparable perovskite-type oxides.