Atomic-scale operando observation of oxygen diffusion during topotactic phase transition of a perovskite oxide

Topotactic phase transition of perovskite oxides enables fast, reversible oxygen transport with minimal volume change, which is advantageous for applications in solid oxide fuel cells. However, the oxygen-diffusion mechanism remains elusive due to the lack of direct atomic-scale observations. Here, we report operando atomic-scale observation and simulation revealing the diffusion mechanism during the topotactic transition of perovskite SrFeO3 to brownmillerite SrFeO2.5. Hyper-stoichiometric brownmillerite phase containing excess oxygen emerges at the phase boundary facilitates oxygen diffusion; oxygen diffuses predominantly along the FeO4 tetrahedral chains via sequential modification of oxygen coordination between FeO4 and FeO5. A steady-state oxygen diffusion is attained through interstitialcy diffusion across the fast-diffusion channels, which accommodates excess oxygen at the interstitial sites between SrO columns. The flexibility of multivalent Fe ions in accommodating various oxygen coordination and the rigidity of Sr lattice framework embracing excess oxygen are key to the fast, anisotropic oxygen diffusion.