Atomic-Scale Observation of Structure Transition from Brownmillerite to Infinite Layer in SrFeO2.5 Thin Films

Stoichiometry modulation in transition-metal perovskite oxides is a pivotal process that can give rise to unprecedented structures and new functionalities. Despite intensive study, the atomic mechanism of the structural transitions occurring in these versatile solid-state ceramics is far from well understood. Here, utilizing in situ electrical biasing (scanning) transmission electron microscopy, we show that the transformation from brownmillerite SrFeO2.5 to infinite-layer SrFeO2 can be directly visualized with atomic resolution. Our observations unveil that the transition process occurring in the cross section thinned film contains two steps. At the initial stage, the tilting of FeO6 octahedra and rotation of the FeO4 tetrahedra due to possible ferroelectricity result in conversion from the mixed structural domains (a(parallel to)| and b(parallel to)) to a homogeneous domain (a(parallel to)) in SrFeO2.5; next, the electric reduced infinite-layer SrFeO2 gradually begins to nucleate at the film/substrate end showing a sharp interface and finally spreads across the entire film. The microscopic kinetics is dominated by the rearrangement of the Fe-O bonding geometry, which occurs through a two-step phase-transition mechanism involving oxygen polyhedral rearrangement, oxygen diffusion, and planar phase formation along the c-axis. This work clarifies the microscopic atomic processes of the oxygen polyhedral rearrangement, oxide ion migration, and the final phase transition from brownmillerite to infinite-layer structure, and offers a new pathway to obtain designed structural materials with specific functionality.

Automated summary

This study reveals the atomic processes of the structural transition from SrFeO2.5 to SrFeO2, involving the rotation of FeO6 octahedra and FeO4 tetrahedra, as well as the nucleation and diffusion processes at the film/substrate interface.