Oxide-ion diffusion in brownmillerite-type Ca₂AlMnO₅₊ from first-principles calculations

Oxide-ion diffusion pathways in brownmillerite oxides Ca₂AlMnO₅ and Ca₂AlMnO_5.5 are systematically investigated using first-principles calculations. These structures reversibly transform into each other by oxidation and reduction. We examine oxide-ion migration in Ca₂AlMnO₅ and Ca₂AlMnO_5.5 using the nudged elastic band method. In the reduced structure (Ca₂AlMnO₅), oxide-ion migration through a vacancy channel is found to have the lowest migration energy barrier, at 0.58 eV. The migration energy barrier of the second-lowest energy path, perpendicular to the vacancy channel, is found to be 0.98 eV. In the oxidized structure (Ca₂AlMnO_5.5), oxide-ion migration within AlO₆ layers has migration energy barriers of 0.55 eV and 0.56 eV in the [100] and [001] directions, respectively. Oxide-ion migration perpendicular to the AlO₆ layer has a migration energy barrier of 1.33 eV, suggesting that oxide-ion diffusion in the [010] direction is difficult even at elevated temperature. These results indicate that diffusion in the reduced phase is predominantly one-dimensional whereas it is two-dimensional in the oxidized phase.