The electronic structure, defect chemistry, and transport properties of members of the mixed ionic electronic conducting SrTi1 - x Fe x O3 - y (STF) solid-solution system are revisited, and an improved defect chemical model is proposed in which Fe is considered to be one of the main constituents that shape the energy-band structure of STF, rather than an impurity dopant with acceptor-like character. As a consequence of the high inherent deficiency in the oxygen sublattice, introduced by the mixed-valence states of the B-site cations Ti4+ and Fe3+, oxygen vacancies and interstitials generated by the anion Frenkel reaction dominate the defect equilibria, leading to predominant ionic conductivity at intermediate partial pressures of oxygen. Increasing Fe content results in both a systematic decrease in band-gap energy, E g 0 = 3.2 − 1.9x + 0.5x 2 eV, and reduction enthalpy, ΔH red = 5.8 − 3.4x + 1.7x 2 eV. The decrease in band gap is explained on the basis of the systematic broadening of the Fe-derived 3d band lying above the O-derived 2p valence band. The Fe-induced changes in band structure were experimentally confirmed by both optical and photoelectron (XPS) spectroscopies.

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