Electronic-structure evolution of SrFeO_3–<i>x</i> during topotactic phase transformation

Oxygen-vacancy-induced topotactic phase transformation between the ABO_2.5brownmillerite structure and the ABO₃perovskite structure attracts ever-increasing attention due to the perspective applications in catalysis, clean energy field, and memristors. However, a detailed investigation of the electronic-structure evolution during the topotactic phase transformation for understanding the underlying mechanism is highly desired. In this work, multiple analytical methods were used to explore evolution of the electronic structure of SrFeO_3-<i>x</i>thin films during the topotactic phase transformation. The results indicate that the increase in oxygen content induces a new unoccupied state of O 2<i>p</i>character near the Fermi energy, inducing the insulator-to-metal transition. More importantly, the hole states are more likely constrained to the<i>dx</i>²-<i>y</i>²orbital than to the<i>d</i>3<i>z</i>²-<i>r</i>²orbital. Our results reveal an unambiguous evolution of the electronic structure of SrFeO_3-<i>x</i>films during topotactic phase transformation, which is crucial not only for fundamental understanding but also for perspective applications such as solid-state oxide fuel cells, catalysts, and memristor devices.