Oxygen Diffusion in Brownmillerite Sr₂Fe₂O₅ is Two-Dimensional: Results from a Molecular Dynamics Study

Electrically insulating A(2)B(2)O(5) brownmillerite materials can be transformed through a reversible, topotactic phase transition to conducting ABO(3-delta) perovskite phases. Such systems are of emerging interest for resistive random-access devices. The key process for the phase transition is oxygen diffusion, but to date, experimental or computational studies yielding oxygen diffusion coefficients in brownmillerite materials are rare. In this study, we use molecular dynamics simulations to directly investigate oxygen tracer diffusion in the brownmillerite Sr2Fe2O5 phase and the SrFeO2.5 perovskite phase. Our results for brownmillerite Sr2Fe2O5 go beyond computed diffusion coefficients: They indicate that oxygen vacancies execute two-dimensional diffusion between the equatorial sites of the FeO6 octahedra, and surprisingly, that oxygen interstitials are not confined to the oxygen-vacancy channels (as widely assumed in the literature) but migrate two-dimensionally by interstitial and interstitialcy mechanisms in the FeO4 layers. Comparisons with experimental data are possible for the perovskite phase, and good agreement is found between simulation and experiment for the oxygen-vacancy diffusivity in terms of both absolute magnitude and activation enthalpy.