Ab initio thermodynamic study of (Ba,Sr)(Co,Fe)O₃ perovskite solid solutions for fuel cell applications

(Ba,Sr)(Co,Fe)O₃ (BSCF) perovskite solid solutions are promising materials for solid oxide fuel cell cathodes and oxygen permeation membranes. Cathode performance strongly depends on the morphology of these materials remaining as a single phase or two-phase mixture. Combining ab initio calculations of the atomic and electronic structure of different supercells with thermodynamics of solid solutions, we have constructed and discussed phase diagrams of several important BSCF chemical compositions. It is demonstrated that in BSC cobaltite solid solution the spinodal decomposition may occur already at relatively low temperatures, while ferrite (BSF and SCF) solid solutions decompose at relatively high temperatures forming a two-phase system where the coexisting hexagonal and cubic phases significantly differ in fractions of constituents.