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

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 SrFe_0.5Co_0.5O_2.5 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 Co^2.91+ to Co^2.00+. 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.

Fig. 1. Structural evolution of SrFe_0.5Co_0.5O_2.5 (SFCO) during thermal reduction.

a Schematic of the reduction process in brownmillerite SFCO thin films on (001) SrTiO₃ via annealing in 3% H₂/Ar forming gas. Green, red, blue, and grey spheres represent Sr, Fe/Co, Ti, and O atoms, respectively. b X-ray diffraction patterns of as-grown and reduced SFCO films. c Out-of-plane lattice constants as a function of annealing temperature (1 h fixed). d Out-of-plane lattice constants as a function of annealing time at 400 °C. Blue and yellow background regions indicate the brownmillerite phase and the oxygen-deficient perovskite phase, respectively, emphasising the temperature range (300–400 °C) where the structural transition occurs.

Fig. 2. Element-specific electronic structure evolution upon reduction.

X-ray absorption spectra (XAS) of (a) Fe L-edge and (b) Co L-edge for as-grown and reduced SrFe_0.5Co_0.5O_2.5 (SFCO) films. c Estimated Fe and Co valence states from multiplet fitting. Lists relative proportions of Fe⁴⁺, Fe³⁺, Fe²⁺ and Co⁴⁺, Co²⁺ in as-grown and 5 h reduced films. d O K-edge XAS spectra for SFCO films before and after reduction, showing changes in hybridization. e O K-edge XAS spectra for SrFe_1−xCo_xO_2.5 with different x values.

Fig. 3. DFT calculations of oxygen vacancy formation.

a Atomic structure of brownmillerite SFCO indicates three inequivalent oxygen sites. In the schematic, green, blue, and red spheres represent Sr, Fe/Co, and O atoms, respectively. b Computed oxygen vacancy formation enthalpies at different oxygen sites and B-site configurations.

Fig. 4. Optical properties of brownmillerite and reduced SFCO films.

a Optical conductivity spectra of SrFe_1−xCo_xO_2.5 films (x = 0, 0.2, 0.5, and 1.0), showing composition-dependent absorption features. b Optical conductivity spectra of SrFe_0.5Co_0.5O_2.5 before and after reduction, highlighting spectral weight suppression and bandgap widening from 2.47 eV to 3.04 eV. Inset: photographs of as-grown (dark and opaque) and reduced (lighter and partially transparent) films on SrTiO₃, visually confirming enhanced transparency upon reduction.

Fig. 5. In situ monitoring of phase reversibility.

a In situ x-ray diffraction and b resistance measurements during sequential 3% H₂/Ar forming gas switching at 400 °C, showing structural reversibility and electronic modulation among three distinct phases.