Sr(Fe,Co)O _3−δ Mixed Conductors: How Cobalt Drives Oxygen Vacancy Formation

In this work, the oxygen nonstoichiometry and local structural features of the SrFe1–xCoxO3−δ (x = 0, 0.2, 0.4, 0.6, 0.8) system, which are mixed ionic–electronic conducting (MIEC) perovskite oxides, were systematically investigated. All synthesized materials exhibited a pseudocubic crystal structure, as confirmed by X-ray diffraction (XRD). Oxygen nonstoichiometry (δ) was determined using iodometric titration and X-ray absorption spectroscopy (XAS) at the Fe and Co K-edges. An increase in oxygen nonstoichiometry was observed with an increasing cobalt content. XAS analysis at the transition metal (TM) K-edges enabled the independent determination of average oxidation states for Fe and Co, revealing Fe to be approximately 3.2+ and Co in the range of 2.9+ to 3.0+. Furthermore, local coordination environments around transition metal ions were predominantly octahedral, supporting the results obtained via XRD. Complementary K-edge XAS measurements provided insights into the spin states of B-site cations, indicating a mixed- and an intermediate-spin state near the surface and a high-spin state in the bulk of the materials. The surface XAS measurements at the L-edge of cobalt confirmed the presence of Co2+ species. The combined high oxygen deficiency, dominant octahedral coordination, and high structural symmetry suggest the absence of long-range vacancy ordering in the SrFe1–xCoxO3−δ materials. Furthermore, they were preferentially located around cobalt ions.