Selective reduction in epitaxial SrFe0.5Co0.5O2.5 and its reversibility

Joonhyuk Lee, Yu-Seong Seo, Krishna Chaitanya Pitike, Gowoon Kim, Sangkyun Ryu, Hyeyun Chung, Su Ryang Park, Sangmoon Yoon, Younghak Kim, Valentino R. Cooper, Hiromichi Ohta, Jinhyung Cho and Hyoungjeen Jeen ()
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Joonhyuk Lee: Pusan National University
Yu-Seong Seo: Sungkyunkwan University
Krishna Chaitanya Pitike: Pacific Northwest National Laboratory
Gowoon Kim: Hokkaido University
Sangkyun Ryu: Pusan National University
Hyeyun Chung: Pusan National University
Su Ryang Park: Gachon University
Sangmoon Yoon: Gachon University
Younghak Kim: Pohang University of Science and Technology
Valentino R. Cooper: Oak Ridge National Laboratory
Hiromichi Ohta: Hokkaido University
Jinhyung Cho: Pusan National University
Hyoungjeen Jeen: Pusan National University

Nature Communications, 2025, vol. 16, issue 1, 1-9

Abstract: Abstract Oxygen-vacancy engineering in transition metal oxides enables programmable functionalities by modulating the valence states and local coordination of constituents. Here, we report the selective reduction of cobalt ions in epitaxial SrFe0.5Co0.5O2.5 thin films under reducing gas environments, while iron ions remain unchanged. X-ray absorption spectroscopy reveals an absorption edge shift of 1.65 eV in the Co L-edge upon reduction, and multiplet simulations estimate a decrease in the average Co valence from Co2.91+ to Co2.00+. This site- and element-specific reduction leads to the formation of a structurally distinct oxygen-deficient phase stabilized by oxygen vacancies at tetrahedral sites, as confirmed by density functional theory. Optical spectroscopy reveals an increase in the bandgap from 2.47 eV to 3.04 eV, accompanied by enhanced transparency. Furthermore, simultaneous in situ diffraction and transport measurements confirm fully reversible redox-driven transitions among three phases: reduced defective perovskite, brownmillerite, and oxygen-rich perovskite phases. These findings demonstrate that selective redox control in multi-cation oxides enables the realization of chemically and functionally distinct oxygen-deficient phases.

Date: 2025
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DOI: 10.1038/s41467-025-62612-1

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