Oxygen-Driven Structural Transformations in (La, Sr)CoO₃ Thin Films

Epitaxial La_1-xSr_xCoO₃ thin films exhibit a rich hierarchy of oxygen-driven phase transformations, enabling systematic tuning between distinct crystal and electronic ground states. We investigate the structural evolution of La_1-xSr_xCoO₃ under varying annealing conditions using in situ and ex situ X-ray diffraction. Starting from a perovskite precursor, partial oxygen loss drives the emergence of the Grenier phase—an intermediate vacancy-ordered structure characterized by alternating CoO₆ octahedra and CoO₄ tetrahedra. Further reduction promotes the formation of the Brownmillerite La_2-xSr_xCo₂O₅ phase. Under stronger reducing conditions, cobalt diffuses toward the surface, leading to the development of layered Ruddlesden–Popper–type configurations. Applying a soluble capping layer enables removal of the diffused cobalt species after transformation and stabilization of the Ruddlesden–Popper phase. These results reveal sequential redox-driven pathways connecting perovskite, Brownmillerite, and Ruddlesden–Popper structures, highlighting the interplay between oxygen stoichiometry, cation mobility, and lattice topology in cobaltate thin films.