Layer Resolved\nCr Oxidation State Modulation in Epitaxial\nSrFe<sub>0.67</sub>Cr<sub>0.33</sub>O<sub>3−δ</sub> Thin\nFilms

Understanding how doping influences physicochemical properties\nof ABO<sub>3</sub> perovskite oxides is critical for tailoring their\nfunctionalities. In this study, SrFe<sub>0.67</sub>Cr<sub>0.33</sub>O<sub>3−δ</sub> epitaxial thin films were used to examine\nthe effects of Fe and Cr competition on structure and B-site cation\noxidation states. The films exhibit a perovskite-like structure near\nthe film/substrate interface, while a brownmillerite-like structure\nwith horizontal oxygen vacancy channels predominates near the surface.\nElectron energy loss spectroscopy shows Fe remains Fe<sup>3+</sup>, while Cr varies from ∼Cr<sup>3+</sup> (tetrahedral layers)\nto ∼Cr<sup>4+</sup> (octahedral layers) within brownmillerite\nphases and becomes ∼Cr<sup>4.5+</sup> in perovskite-like phases.\nTheoretical simulations indicate that Cr–O bond arrangements\nand the way oxygen vacancies interact with Cr and Fe drive Cr charge\ndisproportionation. High-valent Cr cations introduce additional densities\nof states near the Fermi level, reducing the optical bandgap from\n∼2.0 eV (SrFeO<sub>2.5</sub>) to ∼1.7 eV (SrFe<sub>0.67</sub>Cr<sub>0.33</sub>O<sub>3−δ</sub>). These findings offer\ninsights into B-site cation doping in the perovskite oxide framework.