Abstract

Oxygen non-stoichiometry, structural properties, and phase relations in Ca and Sr substituted LaMnO_3 ± δ were studied by means of thermal analysis, X-ray diffraction, and iodometric titration. Hexagonal α-SrMnO_3 − δ and LaMnO_3 ± δ perovskite are only partly miscible. Room temperature metastable La_1 − xSr_xMnO_3 ± δ solid solutions, prepared by quenching from high temperatures where there is complete solid solubility between cubic β-SrMnO_3 − δ and LaMnO_3 ± δ perovskite, have rhombohedral, tetragonal or cubic symmetry dependent on the Sr content. Orthorhombic LaMnO_3 + δ and CaMnO_3 − δ form a stable La_1 − xCa_xMnO_3 ± δ solid solution in the whole composition region. The unit cell volume of La_1 − xSr_xMnO_3 ± δ and La_1 − xCa_xMnO_3 ± δ solid solutions is reduced with decreasing La-content mainly due to the correlated change in average oxidation state from Mn³⁺ to Mn⁴⁺. The oxygen stoichiometry changes from cation deficient (oxygen excess) at high La-content to oxygen deficiency at low La-content. The enthalpy of the phase transitions of CaMnO_3 − δ and the transition from α-SrMnO₃ (hexagonal) to β-SrMnO_3 − δ (perovskite) were obtained by differential thermal analysis. Finally, the physical appearance of first order phase transition between different perovskite-related structures is analysed in terms of the Gibbs phase rule. First order transitions between different structures of a ternary oxide ABO_3 − δ become two-phase regions in a quasi-binary system like ABO_3 − δ–A′BO_3 − δ.