Understanding Structural Incorporation of Oxygen Vacancies in Perovskite Cobaltite Films and Potential Consequences for Electrocatalysis

Owing to their excellent mixed-ionic and electronic conductivity, fast oxygen kinetics, and cost efficiency, layered oxygen-deficient perovskite oxides hold great potential as highly efficient cathodes for solid oxide fuel cells and anodes for water oxidation. Under working conditions, cation ordering is believed to substantially enhance oxygen diffusion while maintaining structural stability owing to the formation of double perovskite (DP), thus attracting extensive research attention. In contrast, the incorporation of oxygen vacancies and the associated vacancy ordering have rarely been studied at the atomic scale, despite their decisive roles in regulating the electronic and spin structures as well as in differentiating the crystal structure from DP. Here, atomicresolution transmission electron microscopy is used to directly image oxygen vacancies and measure their concentration in (Pr,Ba)CoO3-delta films grown on SrTiO3 substrates. We find that accompanied by the presence of oxygen vacancy ordering at Co-O planes, the A-O (A = Pr/Ba) planes also exhibit a breathing-like lattice modulation. Specifically, as confirmed by first-principle calculations, the AO-AO interplanar spacings are found to be linearly correlated with the vacancy concentration in the enclosing Co-O planes. On this basis, potential consequences of oxygen occupancy for the catalytic properties of structurally pure PBCO phases are discussed. Through establishing a simple correlation of oxygen concentration with the easily achievable lattice measurement, our results pave a way for better understanding the structure-performance relationship of oxygen-deficient complex cobaltites used for electrocatalysis.