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

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 Sr1-yFe0.5-xTa0.5+xO3-delta, where x, y >= 0, are thoroughly studied using a combination of experimental and theoretical methods. It is argued that the basic compound SrFe0.5Ta0.5O3-delta 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 Sr1-yFe0.5-xTa0.5+xO3-delta 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 Sr1-yFe0.5-xTa0.5+xO3-delta 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 Sr0.95Fe0.45Ta0.55O3-delta at elevated temperatures appears to be sufficiently enlarged as compared to pristine SrFe0.5Ta0.5O3-delta. Similar to that, the conductive properties of tantalum-enriched phase Sr0.95Fe0.45Ta0.55O3-delta 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.

Automated summary

The crystalline and electronic band structures, thermodynamic stability, oxygen non-stoichiometry and high-temperature transport properties of perovskite-like solid solutions Sr1-yFe0.5-xTa0.5+xO3-delta have been thoroughly studied. It is found that intentionally introducing defects in the strontium sublattice can stabilize the oxides with excess tantalum content. The charge states in the iron sublattice are mainly Fe3+ ions, and the band structure calculations support the semiconducting nature of electrical transport. The overall defect structure heavily relies on oxygen vacancy formation processes. Increasing the tantalum content improves the oxygen non-stoichiometry and conductive properties of the materials.