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 Sr 4 Co 3 O 9, a triple-layered cobalt oxide with a layered brownmillerite structure, as an epitaxial thin film on a LaAlO 3 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 A 2 B 2 O 5, the triple-layered A 4 B 3 O 9 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 Sr 4 Co 3 O 9 is lower compared to other layered cobalt oxides, such as Na x CoO 2 . Our results demonstrate the impact of structural modulation in layered brownmillerite oxides, offering a perspective for designing functional materials.