Epitaxial Growth of Triple-Layered Brownmillerite Cobalt Oxide Sr₄Co₃O₉

Layered perovskite oxides with the Ruddlesden-Popper (RP) structure exhibit diverse functionalities, yet the synthesis of cobalt-based RP oxides is hindered by competing phase formation. In this study, we successfully synthesized Sr₄Co₃O₉, a triple-layered cobalt oxide with a layered brownmillerite structure, as an epitaxial thin film on a LaAlO₃ substrate. Soft X-ray absorption spectra confirmed oxygen nonstoichiometry and a mixed-valence state of Co^3.3+ while scanning transmission electron microscopy revealed the presence of a tetrahedral layer in the central perovskite block. The structural comparison indicates that unlike conventional brownmillerite A₂B₂O₅, the triple-layered A₄B₃O₉ phase features axially elongated octahedra with significant B-site off-centering, driven by Coulomb repulsion from the tetrahedral B cations. Density functional calculations suggest a charge-ordered antiferromagnetic state. The thermal power of Sr₄Co₃O₉ is lower compared to other layered cobalt oxides, such as NaCoO₂. Our results demonstrate the impact of structural modulation in layered brownmillerite oxides, offering a perspective for designing functional materials.