First principles calculations of (Ba,Sr)(Co,Fe)O3 − δ structural stability

First principles total-energy calculations of an ideal BSCF perovskite-type solid solution, the crystal containing basic point defects, and a set of relevant solid–solid solutions are presented. Our DFT modeling of defects (Frenkel, Schottky and cation exchange) and disordering in the BSCF perovskites reveals that the material tends to decompose at relatively low temperatures into a mixture of new perovskite and oxide phases. These new phases are likely to appear at grain boundaries and surface interfaces. This instability is predicted to negate advantages of fast oxygen transport chemistry and impede the applicability of BSCF-based SOFC and ceramic permeation membranes. We discuss possible mechanisms and origins of defect-induced (in)stability in the context of available experiments. This research explains the observed SOFC performance reduction, the significant scattering in the reported degree of oxygen nonstoichiometry, and provides insights on enhancing mass transport and energy conversion in SOFC and oxygen separation ceramic membranes.

► BSCF accommodates defects and disorders that determine its chemical instability.
► Frenkel and Schottky defects are favorable; oxygen vacancy disorder is preferred.
► Oxygen vacancies serve to stabilize the BSCF cubic phase.
► Vacancy formation energy is smaller in cubic phase than in hexagonal phase.
► Energy difference in two phases explains dispersion of δ measured experimentally.