Defect formation, ordering, and transport in SrFe_1–x Si _x O_3–δ (x = 0.05–0.20)

Abstract

Oxygen nonstoichiometry of perovskite-like SrFe_1–x Si _x O_3–δ (x = 0.05–0.20), studied by thermogravimetric analysis and coulometric titration in the oxygen partial pressure range 10⁻²⁰–0.5 atm at 700–950 °C, decreases with Si⁴⁺ additions. The equilibrium ${p}_{{\mathrm{O}}_2}$–T−δ diagrams can be adequately described by a model accounting for anion site-exclusion effects near highly stable SiO₄ tetrahedra and energetic favorability of the defect clusters formed by two tetrahedra sharing one oxygen vacancy. This model was validated by atomistic computer simulations. The standard thermodynamic functions for oxygen incorporation and iron disproportionation reactions are essentially independent of silicon concentration, as for the migration activation energies of the p- and n-type electronic charge carriers. On the contrary, at low temperatures, Si-doping leads to a higher oxygen deficiency, simultaneously suppressing long-range vacancy ordering and increasing oxygen coordination of iron cations as estimated from the Mössbauer spectra. These phenomena are associated, again, with vacancy trapping near randomly distributed Si⁴⁺. The Mössbauer spectroscopy, transmission electron microscopy, and electron diffraction studies showed that Si⁴⁺ substitution progressively reduces the content of brownmillerite-like nanodomains typical for SrFeO₃-based materials.