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The oxygen content was measured in cubic perovskite-type La 1/3 Sr 2/3 Fe 1- x Mn x O 3- δ ( x = 0.1, 0.17, 0.25, and 1/3) in the range of oxygen partial pressure from 10 -22 to 0.5 atm at 750-950 °C with a step of 50 °C by coulometric titration. Gradual removal of oxygen from the oxides during the measurements was carried out until the stability limit was achieved and the reductive decomposition began. An increase in manganese content was shown to lead to a decrease in the stability of La 1/3 Sr 2/3 Fe 1- x Mn x O 3- δ under reducing conditions. The obtained data on oxygen content were used for defect chemistry modeling in the oxides. The enthalpy of the Fe 3+ to Fe 4+ and Mn 3+ to Mn 4+ oxidation reactions (Δ H ox 0 ) was determined to be -103.2 ± 0.3 and -250 ± 2 kJ mol -1 , respectively, for the x = 0.1 composition, and increased slightly with increasing manganese content. The large difference in Δ H ox 0 determines a strong distinction between the behavior of iron and manganese in perovskite-type oxides. An increase in manganese content in La 1/3 Sr 2/3 Fe 1- x Mn x O 3 -δ was found to lead to a decrease in the concentration of Fe 4+ ions, but did not affect the concentration of Fe 2+ ions. The impact of La/Sr ratio was evaluated by comparison of the obtained data with that for La 0.5 Sr 0.5 Fe 1- x Mn x O 3 -δ , and found to be different for iron and manganese. An increase in lanthanum fraction causes a decrease in the concentration of Fe 2+ ions and an increase in the concentration of Mn 2+ under reducing conditions.

Perovskite-type La 0.5 Sr 0.5 Fe 0.9 Mo 0.1 O 3- 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 La 0.5 Sr 0.5 Fe 0.9 Mo 0.1 O 3- 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 cm 2 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 La 0.5 Sr 0.5 FeO 3- 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.

The oxygen content in La 0.5 Sr 0.5 Fe 1- x Mn x O 3- δ , measured by coulometric titration in a wide range of oxygen partial pressure at various temperatures, was used for defect chemistry analysis. The obtained data were well approximated by a model assuming defect formation in La 0.5 Sr 0.5 Fe 1- x Mn x O 3- δ via Fe 3+ and Mn 3+ oxidation reactions and charge disproportionation on Fe 3+ and Mn 3+ ions. The partial molar enthalpy and entropy of oxygen in La 0.5 Sr 0.5 Fe 1- x Mn x O 3- δ obtained by statistical thermodynamic calculations were found to be in satisfactory agreement with those obtained using the Gibbs-Helmholtz equations, thus further confirming the adequacy of the model. The impact of manganese substitution on defect equilibrium in La 0.5 Sr 0.5 Fe 1- x Mn x O 3- δ was shown to be attributed to a lower enthalpy of Mn 3+ oxidation reaction ( vs. for the oxidation of Fe 3+ ) and the charge disproportionation reaction on Mn 3+ ( vs. for that on Fe 3+ ). The former makes Mn 4+ ions more resistant to reduction than Fe 4+ . The latter favors the presence of Mn 2+ , Mn 3+ , and Mn 4+ ions in oxides in comparable concentrations. The distribution of charge carriers over iron and manganese ions was determined as a function of oxygen content in La 0.5 Sr 0.5 Fe 1- x Mn x O 3- δ .

The oxygen content and electrical conductivity in Nd 0.25 Sr 0.75 FeO 3-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 Fe 3+ 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. Nd 0.25 Sr 0.75 FeO 3-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.