Defect equilibria and partial molar properties of (La,Sr)(Co,Fe)O3−δ

The oxygen nonstoichiometry δ of La1−xSrxCo1−yFeyO3−δ (x = 0.6 and y = 0.2, 0.4) was investigated by thermogravimetry in the range 703 ≤ T/°C ≤ 903 and 1E−5 < pO2/atm < 1. The oxygen deficit increases with increasing T and decreasing pO2. Electronic conductivities σ were measured as a function of pO2 in the range 1E−5 < pO2/atm < 1 at 700 ≤ T/°C ≤ 900. At constant T, a p-type pO2-dependence of σ is observed. Oxygen nonstoichiometry data are analyzed with regard to the enthalpy and entropy of oxidation ΔHoxθ and ΔSoxθ, as well as to the partial molar enthalpy and entropy of oxygen with respect to the standard state of oxygen (pO2θ = 1 atm), (hO − HOθ) and (sO − SOθ), respectively. For 2.67 ≤ (3 − δ) ≤ 2.79, (hO − HOθ) decreases with increasing δ, while (sO − SOθ) is constant within the limits of error. Defect chemical modelling was performed by an ideal solution model under consideration of three different valence states for B-site ions (Co or Fe). The dependence of σ on δ is modelled, using calculated defect concentrations as functions of δ. Deviations from the ideal behaviour suggest an immobilization of n-type charge carriers by oxygen vacancies.