The influence of strontium deficiency on thermodynamics of defect formation, structural stability and electrical transport of SrFe0.5Ta0.5O3−δ-based solid solutions with an excess tantalum content

Abstract

The crystalline and electronic band structures, thermodynamic stability, oxygen non-stoichiometry and high-temperature transport properties of perovskite-like solid solutions with a general formula Sr_1−yFe_0.5−xTa_0.5+xO_3−δ, where x, y ≥ 0, are thoroughly studied using a combination of experimental and theoretical methods. It is argued that the basic compound SrFe_0.5Ta_0.5O_3−δ possesses an orthorhombic lattice symmetry, while its tantalum-doped derivatives belong to a tetragonal space group. Importantly, the purposeful addition of a certain deficiency in a strontium sublattice is shown to be a valid method for stabilizing the Sr_1−yFe_0.5−xTa_0.5+xO_3−δ oxides with an excess tantalum content. Detailed studies of charge states in an iron sublattice suggest the predominance of Fe³⁺ ions even in tantalum-enriched materials. Also, the band structure calculations support the semiconducting nature of electrical transport with localized n-type conductivity provided by small polarons represented by Fe²⁺ ions. The overall defect structure of Sr_1−yFe_0.5−xTa_0.5+xO_3−δ compounds is proved to heavily rely on oxygen vacancy (V_O) formation processes; in turn, the presence of strontium vacancies is shown to be an important factor that can decrease the respective energy penalties to introduce V_O defects in the lattice. As a result, the experimentally measured oxygen non-stoichiometry for Sr_0.95Fe_0.45Ta_0.55O_3−δ at elevated temperatures appears to be sufficiently enlarged as compared to pristine SrFe_0.5Ta_0.5O_3−δ. Similar to that, the conductive properties of tantalum-enriched phase Sr_0.95Fe_0.45Ta_0.55O_3−δ are shown to be improved. On the basis of the obtained results, it is argued that cation non-stoichiometry is a valuable tool for enhancing thermodynamic and transport characteristics of perovskite-like compounds, which are currently viewed as promising materials for high-temperature applications.