PNNL

Abstract

Understanding how doping influences physicochemical properties of ABO3 perovskite oxides is critical for tailoring their functionalities. This study uses SrFe0.67Cr0.33O3-d (SFCO) epitaxial thin films as a model system to examine how Fe and Cr competition affects crystalline structure and B-site cation oxidation states. SFCO films exhibit a perovskite-like structure near the film/substrate interface, while a brownmillerite structure with horizontal oxygen vacancy channels predominates near the film/vacuum interface. Electron energy loss spectroscopy reveal that Fe3+ is consistently present, while the Cr oxidation state varies from ~Cr3+ (tetrahedral layers) to ~Cr4+ (octahedral layers) in brownmillerite phase regions and becomes ~Cr4.5+ in perovskite-like phase regions. Theoretical simulations indicate that the Cr-O bond arrangements drive the disproportionation of Cr ionic charge. Additionally, high-valent Cr ions introduce additional Cr 3d states near the Fermi level, reducing the optical bandgap compared to SrFeO2.5. These findings enhance our understanding of B-site cation doping in perovskite oxide framework.