Thermodynamic stability of perovskites and related compounds in some alkaline earth-transition metal-oxygen systems

The thermodynamic properties of some alkaline earth (A)-transition metal (M) perovskites and K₂NiF₄ compounds have been collected, analyzed, and utilized to examine their stabilities by constructing the chemical potential diagrams of a log [ a(A) a(M)] vs log P(O₂) plot. A thermodynamic analysis was performed on the dissociation reaction of K₂NiF₄ compounds (A₂MO₄) into perovskites (AMO₃) and alkaline earth oxides (AO) using empirical correlations between stabilization energy and tolerance factor. It has been found that the softness of calcium ions, which shrink markedly with decreasing coordination number from 12 to 9, makes the calcium K₂NiF₄ compounds (Ca₂MO₄) relatively less stable with increasing radius of the transition metal ions, r(M⁴⁺). This destabilization related to the coordination-number-dependent radii implies that when compared with the strontium perovskites, the calcium analogous perovskites may have a smaller number of oxygen vacancies, because the formation of oxygen vacancies should be accompanied with a decrease in coordination number of A-site ions.