Oxygen stoichiometry, unit cell volume, and thermodynamic quantities of perovskite-type oxides

Abstract

Perovskite-type oxides ${\text{A}}_{{{\text{1 - }}a}} {\text{A}}^{\prime }_{{\text{a}}} {\text{B}}_{{{\text{1 - }}b}} {\text{B}}^{\prime }_{{\text{b}}} {\text{O}}_{{{\text{3 - }}x}}$ with A, A′=La, Ba, Sr; B, B′=Mn, Fe, Co were investigated by means of thermal analysis, solid electrolyte cells, and X-ray diffraction. Partial molar thermodynamic quantities are determined and their relations with O/M stoichiometry, unit cell volume, and phase stability were studied. The absolute values of partial molar enthalpies of perovskite-type oxides increase with increasing O/M stoichiometries and with decreasing unit cell volumes of the cubic perovskite-type structure, corresponding to higher chemical stabilities. The substitution of Ba for La, Ba for Sr, Co for Fe, and Fe for Mn lead to increase in unit cell volumes and decrease in absolute values of ΔH ⁰. The ΔH ⁰ values of the cobaltites/ferrites range from −33.5 kJ/mol for SrCo_0.8Fe_0.2O_3−x to −72.5 kJ/mol for La_0.2Sr_0.8Co_0.6Fe_0.4O_3−x, and of the manganates up to −132 kJ/mol for Ca_0.5Sr_0.5Mn_0.8Fe_0.2O_3−x .