Synthesis and Characterization of Strontium Cobaltite Membranes Under Topotactic Transformations

Strontium cobaltite SrCoO_3-δ (SCO) is known for its multiple electronic and magnetic ground states, tunable by Co oxidation number and a defect-induced structural phase transition. Its relatively low oxygen vacancy formation energy and high oxygen vacancy conductivity put the material on the verge of structural instability; a small amount of external stimulus can trigger topotactic transformation between the perovskite SrCoO₃ and the brownmillerite SrCoO_2.5 phases by either the insertion or extraction of oxygen ions. Understanding this topotactic transformation process is of great interest for functional oxides research ranging from new electromagnetic ground states in reduced crystal structures to applications in electrochemical energy conversion processes.<br/><br/>Complex oxide membranes provide an ideal material platform to characterize and manipulate the topotactic transformation, where their freestanding geometry is advantageous in elastic strain applications as well as atomic-scale imaging by transmission electron microscopy. Here, we report the successful synthesis of mm-scale SCO thin film membranes by etching a water-soluble sacrificial layer of (Ca,Sr)₃Al₂O₆ in oxide thin film heterostructures. Both SrCoO_2.5 and SrCoO₃ can be stabilized in freestanding membrane form by tuning the oxygen stoichiometry. This presentation will also discuss the evolution of their local structures and electromagnetic properties upon topotactic transformation, as well as their responses to strains and radiation exposure.