Strong anisotropy and its electric tuning for brownmilleriteSrCoO2.5films with different crystal orientations

Abstract

Brownmillerite oxides ($A\mathrm{B}{\mathrm{O}}_{2.5}$) with long-range ordering of oxygen vacancies own a distinct superstructure formed by alternately stacked octahedral $\mathrm{B}{\mathrm{O}}_{6}$ and tetrahedral $\mathrm{B}{\mathrm{O}}_{4}$ planes. The one-dimensional oxygen vacancy channels within $\mathrm{B}{\mathrm{O}}_{4}$ layers usually lead to high ionic conductivity of brownmillerite oxides, demonstrating great application potential in solid-oxide fuel cells, the oxygen separation membrane, and catalyzers. Here, high quality brownmillerite-$\mathrm{SrCo}{\mathrm{O}}_{2.5}$ films have been epitaxially grown on differently oriented substrates by pulsed laser deposition. The anisotropic structural and physical properties of (110)- and (111)-oriented $\mathrm{SrCo}{\mathrm{O}}_{2.5}$ films were systematically investigated. We found, unlike the out-of-plane (001)-oriented $\mathrm{SrCo}{\mathrm{O}}_{2.5}$ films, the $\mathrm{Co}{\mathrm{O}}_{6}$ and $\mathrm{Co}{\mathrm{O}}_{4}$ planes would alternately stack along one of the [100] and [010] axes for (110)-oriented films and one of the [100], [010], and [001] axes for (111)-oriented films, forming coexisting crystal domains with different orientations. More importantly, the superstructure of these films could be reversibly tuned by alternately applying an electric field along two orthogonal directions, switching between an ordered and a disordered state. Corresponding to structural anisotropy, strong in-plane electronic anisotropy of the (110)-oriented $\mathrm{SrCo}{\mathrm{O}}_{2.5}$ film was revealed, which was also electrically tunable like the superstructure. This work demonstrates the approaches to modify the ionic conduction channels of brownmillerite oxides, opening avenues towards electrically tunable oxygen separation membrane and catalyzers.