Atomic Scale imaging of Topotactic Phase Transitions in Freestanding Strontium Cobaltite Membranes

Topotactic phase transitions (TPTs) are structural phase changes that involve the significant loss or gain of oxygen atoms while preserving the crystalline framework of the lattice. In perovskite oxides, TPTs have recently emerged as innovative pathways for designing complex oxide materials. These transitions induce dramatic alterations in metal oxidation states and atomic coordination, enabling a wide range of electromagnetic ground states, from magnetic ordering to unconventional superconductivity. Moreover, the reversible nature of TPTs holds promising implications for practical applications such as neuromorphic computing devices, garnering significant attention in both fundamental physics and device applications.<br/><br/>Despite the growing research interest in TPTs in recent years, the microscopic details of these transitions, encompassing both structural changes and ionic transport, remain largely unexplored. This is partly due to the constraints imposed by the thin film heterostructure geometry, which limits our ability to directly visualize the transition through conventional means. In this report, we present recent advancements in freestanding oxide membranes that enable the local nature of TPTs to be visualized using electron microscopy. As a model system to comprehend the local structure of TPTs, we examined strontium cobaltite (SrCoO_3-δ), which can transition between a ferromagnetic metal (perovskite) and an antiferromagnetic insulator (brownmillerite). Transmission electron microscopy studies of SrCoO_3-δ membranes unveiled the formation of anisotropic domains in the midst of the transition. Furthermore, atomic-scale images revealed the intricate nature of strains and crystallographic symmetries that dictate the domain patterns. Freestanding oxide membranes offer an ideal platform for in-situ and in-operando studies of TPTs, providing insights into the predictable design of switching phenomena in oxide materials.