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

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 (SrFeO_3-x) using in-situ high-resolution transmission electron microscopy combined with molecular dynamics (MD) simulations. Reduction of SrFeO₃ at 450 °C proceeds through SrFeO_2.5 and a previously unreported vacancy-ordered SrFeO_2+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 SrFeO_2+σ, SrFeO_2.75, and SrFeO_2.875. 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.