Atomic Modeling and Electronic Structure of Mixed Ionic-Electronic Conductor SrTi_{1- x}Fe_xO_{3- x/2+δ} Considered as a Mixture of SrTiO₃ and Sr₂Fe₂O₅

As mixed ionic-electronic conductors (MIECs), ABO₃ perovskite oxides and derivative compounds are candidates for applications such as solid oxide fuel and electrolysis cells. Understanding the atomic configuration and electronic structure of MIECs is important because they form the basis of ionic and electronic conductivities; but it is challenging because the materials tend to be non-dilute systems with substantial partial occupancies of the perovskite sublattices. In this work, we present a computational model for Fe-substituted SrTiO₃ (STF, SrTi_1-xFe_xO_3-x/2+δ), a representative perovskite-derivative MIEC, by considering it as a mixture of perovskite SrTiO₃ (x = 0, δ = 0) and brownmillerite Sr₂Fe₂O₅ (x = 1, δ = 0). Our model accounts for disorder in the form of Ti and Fe species on the perovskite B-site sublattice and of O atoms and vacancies V_O on the oxygen sublattice. The defect chemistry and electronic structure of STF across the full composition range 0 ≤ x ≤ 1 and for small |δ| is addressed within the framework of first-principles density functional theory. We find that this model of the STF solid solution reproduces experimentally known features such as short-range ordering between Fe and V_O and thermodynamic influence on the oxygen content in STF. We illustrate how the electronic structure of STF systematically evolves from the characteristics of end-member compounds, SrTiO₃ and Sr₂Fe₂O₅, and establish the effects of nonzero δ on the type of electronic carriers present. These findings confirm that the SrTi_1-xFe_xO_3-x/2+δ framework is a useful description for this material system, providing a systematic understanding of structure-property relations in the nondilute perovskite mixture. Although this work focuses on STF, the underlying approach may be applicable to other mixtures of perovskite oxides and ordered oxygen vacancy compounds.