Phase stability of BaCo1−yFeyO3−δ by first principles calculations

Free energies of the hexagonal phase and perovskite phases of BaCo1−yFeyO3−δ, a mixed ionic and electronic conductor, are calculated using first principles method. The calculated structure of a known phase is in good agreement with experiment. It is found that the hexagonal phase is most stable at low temperature. As temperature increases, a phase transition to tetragonal perovskite, in which the B site cation valence is reduced to 3+ by release of oxygen, takes place. Further increase of temperature results in the reduction of the perovskite phase, i.e., the valence of the B site cation is reduced to 2+. The temperature of the hexagonal to perovskite phase transition is lowered by mixing Co and Fe, in spite of the increased phonon scattering, due to localization of oxygen vacancies.