Layer Resolved Cr Oxidation State Modulation in Epitaxial SrFe_0.67Cr_0.33O_3-δ Thin Films

Understanding how doping influences physicochemical properties of ABO₃ perovskite oxides is critical for tailoring their functionalities. In this study, SrFe_0.67Cr_0.33O_3-δ epitaxial thin films were used to examine the effects of Fe and Cr competition on structure and B-site cation oxidation states. The films exhibit a perovskite-like structure near the film/substrate interface, while a brownmillerite-like structure with horizontal oxygen vacancy channels predominates near the surface. Electron energy loss spectroscopy shows Fe remains Fe³⁺, while Cr varies from ∼Cr³⁺ (tetrahedral layers) to ∼Cr⁴⁺ (octahedral layers) within brownmillerite phases and becomes ∼Cr^4.5+ in perovskite-like phases. Theoretical simulations indicate that Cr-O bond arrangements and the way oxygen vacancies interact with Cr and Fe drive Cr charge disproportionation. High-valent Cr cations introduce additional densities of states near the Fermi level, reducing the optical bandgap from ∼2.0 eV (SrFeO_2.5) to ∼1.7 eV (SrFe_0.67Cr_0.33O_3-δ). These findings offer insights into B-site cation doping in the perovskite oxide framework.