Oxygen defect formation and migration in Sr2FeO4−δ: Insights from first principles DFT calculations with the PBE+U functional

Abstract

Sr2FeO4−δ is a first-order Ruddlesden-Popper perovsike-type material, prospective as a positrode (O-electrode) material for solid oxide and protonic ceramic electrochemical cells. By means of DFT+Ueff, oxygen vacancies (VO••) and interstitials (Oi″) have been modelled in a broad oxygen stoichiometry range of δ=0.125 to 0.5. For each δ value, all symmetry inequivalent configurations were identified, yielding the VO•• formation energy in the range of 2.0–2.4 eV, almost independent of δ (unlike for simple perovskites). We assign the individual Fe charge states in Sr2FeO4−δ (“oxidation state map”) based on magnetic moment and average Fe-O distance. Interestingly, VO•• and Fe3+ energetically prefer not to be nearest neighbors. Regarding oxygen interstitial species, a Sr-O22−-Fe defect is the most stable form. Oxygen interstitial transport mechanism occurs predominantly within the SrO layer with the intermediate formation of a single oxide Oi2− and an average barrier of 0.5 eV. Oxygen vacancy migration is preferred along a zig-zag path within the FeO2 layer with an effective barrier of 0.6 eV. The comprehensive analysis of Sr2FeO4−δ—serving as prototype also for higher Ruddlesden-Popper phases—including oxidation state map and oxygen defect migration, paves the way for coming investigations of the protonated materials.