Topotactic Phase Transformations in Freestanding Strontium Cobaltite Membranes*

SrCoO_3-δ (SCO) is known for multiple electronic and magnetic ground states tunable by its Co valency and defect-induced topotactic phase transitions (TPT). Its relatively low oxygen vacancy formation energy and high oxygen vacancy conductivity place it on the verge of structural transition; an external stimulus can readily trigger the TPT between the perovskite SrCoO₃ and brownmillerite SrCoO_2.5 phases by insertion/extraction of oxygen ions. This transformation is of great interest for functional oxides research ranging from new electromagnetic ground states in reduced crystal structures to applications in resistive switching.

Complex oxide membranes provide an ideal system to characterize and manipulate the TPT, where their freestanding geometry facilitates imaging by transmission electron microscopy (TEM). Here, we report the successful synthesis of SrTiO₃/SCO/SrTiO₃ membrane heterostructures by etching a water-soluble sacrificial layer of CaSr₂Al₂O₆. Electron and X-ray diffraction, along with high-resolution TEM, X-ray photoelectron spectroscopy, and magnetometry confirm that SrCoO_2.5 and SrCoO₃ membranes can be stabilized via annealing in either reducing or oxidizing environments. This unique material platform enables the observation of TPTs using in-situ characterization methods.