Oxide-ion diffusion in brownmillerite-type Ca 2 AlMnO 5+ δ from first-principles calculations.

Oxide-ion diffusion pathways in brownmillerite oxides Ca 2 AlMnO 5 and Ca 2 AlMnO 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 Ca 2 AlMnO 5 and Ca 2 AlMnO 5.5 using the nudged elastic band method. In the reduced structure (Ca 2 AlMnO 5 ), 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 (Ca 2 AlMnO 5.5 ), oxide-ion migration within AlO 6 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 AlO 6 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.