Phase Transition Regulation and Properties of Transition Metal Oxide SrCoOx

Transition metal oxide SrCoOx (2.5 ≤ x ≤ 3.0) has attracted considerable attention owing to its reversible topotactic phase transition between the brownmillerite SrCoO2.5 phase (BM-SCO, an antiferromagnetic insulator) and the perovskite SrCoO3−δ phase (PV-SCO, a ferromagnetic metal). This phase transition is driven by continuous changes in oxygen stoichiometry and is accompanied by dramatic reconstruction of the valence state, spin state, and electronic ground state of cobalt ions, giving rise to order-of-magnitude differences in magnetic, electrical, optical, and thermal properties. This review systematically summarizes the crystal structures, physical properties, and regulation rules of oxygen vacancy channels (OVCs) in BM-SCO and PV-SCO. It summarizes recent advances in phase-transition regulation strategies, including ionic liquid gating, epitaxial strain engineering, topotactic redox, protonation/hydrogen intercalation, and triboelectric/mechanical-energy induction, with emphasis on frontier achievements such as in situ imaging of freestanding membranes, reduction-mediated oxidation, stepwise oxidation by water vapor, and neutron diffraction determination of magnetic order. Furthermore, the application potential is discussed in terms of resistive switching and memristive behavior, magnetoionics, optical electrochromism, and neuromorphic computing, along with key challenges and future directions concerning cycling stability, room-temperature ionic migration rate, and multi-parameter synergistic regulation.