Unraveling Doping Effects on the Structure and Electronic Properties of Perovskite Oxides

The Science

Doping, the process of adding small amounts of foreign atoms to a material, is an effective way of adjusting material properties, such as conductivity, magnetism, and catalytic activity. However, the effects of doping, particularly how it alters prevalent defects, are still not fully understood. Researchers revealed a species-selective charge modulation in synthesized oxygen-deficient SrFe_0.67Cr_0.33O_3-δ (SFCO) thin films. They observed a clear charge disproportionation in the added chromium (Cr) atoms, with a mixture of +3, +4, and +5 oxidation states. The Cr oxidation state varied across the layers of the film—a result of the oxygen gradient. In contrast, the iron (Fe) atoms remained as Fe³⁺ throughout the film. 

The Impact

This research significantly advances the scientific understanding of how B-site cation doping in ABO₃-structured perovskite oxides can be used to tailor material properties. Perovskite oxides have a wide range of potential applications, including superconductors, memristors, fuel cells, and electrocatalysts. By revealing how the incorporation of Cr cations influences structure and oxidation states, this study provides valuable insight into how doping can control oxygen vacancy distribution and electronic structure. The ability to modulate oxidation states and change the material’s optical bandgap opens new possibilities for improving device performance. 

Summary

This research highlights the critical role of Fe and Cr cation interactions in shaping the structural, electronic, and optical properties of SFCO thin films synthesized by molecular beam epitaxy. The SFCO films exhibit a dual-phase structure, with a perovskite-like phase near the substrate interface and a Brownmillerite-like phase with oxygen vacancy channels near the surface. While Fe maintains a stable Fe³⁺ oxidation state throughout the film, Cr shows a layer-dependent oxidation state. The Cr oxidation state varies between Cr³⁺ and Cr⁴⁺ in the brownmillerite phase and reaches up to Cr^4.5+ in the perovskite-like regions. Theoretical simulations reveal that these variations are driven by the local Cr-O bond environment, which influences the electronic structure and reduces the optical bandgap. These findings provide a deeper understanding of B-site cation doping in perovskite oxide frameworks and demonstrate selective oxidation behavior in multivalent cation systems. This opens new possibilities for designing advanced materials for energy-related applications, including sensors, solid oxide fuel cells, and photovoltaic devices, where tailored electronic and optical characteristics are essential for enhanced performance. 

Contact

Yingge Du, Pacific Northwest National Laboratory, yingge.du@pnnl.gov 

Funding

This work was supported by the Department of Energy (DOE), Office of Science, Basic Energy Sciences, Division of Materials Sciences and Engineering, Synthesis and Processing Science Program, FWP 10122. Part of the electron microscopy work is carried out by using instruments that are funded in part by a grant from the Washington State Department of Commerce’s Clean Energy Fund. Cr L-edge, Fe L-edge, and O K-edge XAS measurements were performed at the Advanced Light Source of Lawrence Berkeley National Laboratory, which is supported by the Director, Office of Science (SC), Office of Basic Energy Sciences, DOE under contract no. DE-AC02-05CH11231. This research used resources of the National Energy Research Scientific Computing Center, a DOE SC user facility supported under Contract No. DE-AC02- 05CH11231 using NERSC Award No. BES-ERCAP0021800.