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 Sr1-yFe0.5-xTa0.5+xO3-δ, 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-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 Sr1-yFe0.5-xTa0.5+xO3-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 Sr1-yFe0.5-xTa0.5+xO3-δ compounds is proved to heavily rely on oxygen vacancy (VO) 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 VO defects in the lattice. As a result, the experimentally measured oxygen non-stoichiometry for Sr0.95Fe0.45Ta0.55O3-δ at elevated temperatures appears to be sufficiently enlarged as compared to pristine SrFe0.5Ta0.5O3-δ. Similar to that, the conductive properties of tantalum-enriched phase Sr0.95Fe0.45Ta0.55O3-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.
ABSTRACT
Perovskite-type La0.5Sr0.5Fe0.9Mo0.1O3-δ synthesized via glycine nitrate combustion and sintered at 1350 °C was found to have an orthorhombic lattice, which transforms upon heating into a rhombohedral and then a cubic one. The oxygen content and electrical conductivity in this oxide were measured in the range of oxygen partial pressures from 10-20 to 0.5 atm at 750-950 °C by coulometric titration and four-probe dc techniques, respectively. The oxygen content data were used to model the defect equilibrium in the oxide. Oxidation, charge disproportionation and electron exchange reactions between iron and molybdenum were assumed by the model to be involved in the formation of defects. The experimental data were well approximated with the model and the concentrations of charge carriers in La0.5Sr0.5Fe0.9Mo0.1O3-δ were determined to be used for the electrical conductivity analysis. The average mobility of oxygen ions and n- and p-type charge carriers was determined to be about 10-5, 0.007, and 0.07 cm2 V-1 s-1 with an activation energy of 0.80 ± 0.02, 0.34 ± 0.01, and 0.23 ± 0.01 eV, respectively. Comparison with La0.5Sr0.5FeO3-δ shows that 10% Mo substitution provides a substantial increase in both the concentration and mobility of n-type carriers, which results in an almost threefold increase in electron conductivity under reducing conditions, while maintaining a high level of ionic conductivity.
ABSTRACT
The oxygen content and electrical conductivity in Nd0.25Sr0.75FeO3-δ were measured in the range of oxygen partial pressure from 10-19 to 0.5 atm at 750-950 °C by coulometric titration and four-probe dc techniques. The thermodynamic analysis of defect equilibrium in the oxide allowed successful simulation of the oxygen content data and calculation of charge carrier concentrations that were used for the analysis of electrical conductivity. The electrical conductivity data were accurately described in the models, which implied that the hole mobility increased upon an increase in the oxygen content in the oxide. The results suggest that only some of the Fe3+ sites are available for hole transport, and their fraction increases with an increase in the oxygen content. The migration energy for oxygen ions, electrons and holes was found to be 0.89 ± 0.02, 0.62 ± 0.01 and 0.230 ± 0.006 eV, respectively. Nd0.25Sr0.75FeO3-δ was shown to have a considerable oxygen conductivity (0.12 S cm-1 at 950 °C) and fairly good stability under reducing conditions, which is a good recommendation for using this oxide as a functional material in high-temperature electrochemical applications.
