The influence of strontium deficiency on thermodynamics of defect formation, structural stability and electrical transport of SrFe 0.5 Ta 0.5 O 3- δ -based solid solutions with an excess tantalum content.

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- y Fe 0.5- x Ta 0.5+ x O 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 0.5 Ta 0.5 O 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- y Fe 0.5- x Ta 0.5+ x O 3- δ oxides with an excess tantalum content. Detailed studies of charge states in an iron sublattice suggest the predominance of Fe 3+ 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 Fe 2+ ions. The overall defect structure of Sr 1- y Fe 0.5- x Ta 0.5+ x O 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 0.95 Fe 0.45 Ta 0.55 O 3- δ at elevated temperatures appears to be sufficiently enlarged as compared to pristine SrFe 0.5 Ta 0.5 O 3- δ . Similar to that, the conductive properties of tantalum-enriched phase Sr 0.95 Fe 0.45 Ta 0.55 O 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.