The effect of temperature and oxygen partial pressure on the concentration of iron and manganese ions in La1/3Sr2/3Fe1−xMnxO3−δ

Abstract

The oxygen content was measured in cubic perovskite-type La_1/3Sr_2/3Fe_1−xMn_xO_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_1/3Sr_2/3Fe_1−xMn_xO_3−δ under reducing conditions. The obtained data on oxygen content were used for defect chemistry modeling in the oxides. The enthalpy of the Fe³⁺ to Fe⁴⁺ and Mn³⁺ to Mn⁴⁺ oxidation reactions (ΔH_ox⁰) 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 ΔH_ox⁰ determines a strong distinction between the behavior of iron and manganese in perovskite-type oxides. An increase in manganese content in La_1/3Sr_2/3Fe_1−xMn_xO_3−δ was found to lead to a decrease in the concentration of Fe⁴⁺ ions, but did not affect the concentration of Fe²⁺ ions. The impact of La/Sr ratio was evaluated by comparison of the obtained data with that for La_0.5Sr_0.5Fe_1−xMn_xO_3−δ, and found to be different for iron and manganese. An increase in lanthanum fraction causes a decrease in the concentration of Fe²⁺ ions and an increase in the concentration of Mn²⁺ under reducing conditions.