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 SrCoO_2.5 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.