Atomic-Scale Visualization of Vacancy-Ordered Intermediates and Pathway Selection in the Perovskite-Infinite-Layer Transformation.

Abstract

Phase transitions in transition metal oxides involving oxygen exchange underpin functionalities ranging from ionic electronics to infinite-layer superconductivity, yet pathway selection and transient intermediates remain difficult to capture. Here, we resolve the atomistic dynamics of oxygen-mediated topotactic transformations in strontium ferrite (SrFeO3-x) using in-situ high-resolution transmission electron microscopy combined with molecular dynamics (MD) simulations. Reduction of SrFeO3 at 450 °C proceeds through SrFeO2.5 and a previously unreported vacancy-ordered SrFeO2+σ phase that retains a less compact three-dimensional Fe-O framework and acts as a structural/chemical buffer before collapse to infinite-layer SrFeO2. During reverse oxidation, the pathway bifurcates: at 200 °C a direct interface-driven phase front converts the SrFeO2 film to perovskite SrFeO3, whereas deep undercooling to 20 °C induces a spatiotemporal competition between the SrTiO3 substrate and ambient oxygen reservoirs that stabilizes a cascade of vacancy-ordered intermediates, including SrFeO2+σ, SrFeO2.75, and SrFeO2.875. MD simulations further reveal a crossover from cooperative, stepwise oxygen diffusion to more continuous oxygen incorporation, rationalizing when staged metastable trapping versus near-direct transformation occurs. Density functional theory calculations show that these trapped intermediates are electronically and magnetically distinct phases rather than merely structural stepping stones, underscoring nonequilibrium pathway selection as a route to functional states beyond the equilibrium phase diagram.