Strain Tuning the Kinetics of Topotactic Phase Transitions in Epitaxial Oxide Films.

We report the kinetics of the topotactic brownmillerite-perovskite phase transition in epitaxial strontium cobaltite (SrCoO x ) thin films. Our findings reveal asymmetric switching kinetics between the oxygen-deficient SrCoO2.5 and oxygen-rich SrCoO3 phases, driven by the epitaxial strain state. Using phase-field modeling, we demonstrate that nucleation and growth dynamics are governed by local oxygen concentration-dependent diffusivity and surface oxygen flux, both of which are strain-dependent. The perovskite phase exhibits higher strain tunability of these parameters compared to the brownmillerite phase, explaining the divergent kinetic pathways upon changes in epitaxial strain. Specifically, transitions on (LaAlO3)0.3(Sr2TaAlO6)0.7 (001) substrates are abrupt and nucleation-dominated, characteristic of order-disorder transformations. Conversely, transitions on SrTiO3 (001) substrates are spatially uniform, with phase propagation originating from the film interface, characteristic of diffuse transformations. These results provide a framework for controlling topotactic transformation rates through strain engineering in functional oxides.