Effect of Mn valence on crystal structure of La–Mn–O perovskite oxides

La:Mn = 1:1 mixtures of lanthanum oxide and manganese carbonate were heat-treated under various oxygen partial pressures at 1400 °C or 1300 °C. The Mn valence of the samples was measured by a chemical analysis, and the crystal structures were refined by the powder x-ray diffraction and the Rietveld analysis. A novel orthorhombic perovskite phase, belonging to the space group Pbnm and containing - Mn²⁺ ions, was formed by heat-treatment under low oxygen partial pressures. The structure was very close to a cubic symmetry. It is supposed that the micro Jahn-Teller effects of Mn²⁺ ions were nearly canceled by one another.