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

The oxygen nonstoichiometry δ of La_1−xSr_xCo_1−yFe_yO_3−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 ΔH_ox^θ and ΔS_ox^θ, as well as to the partial molar enthalpy and entropy of oxygen with respect to the standard state of oxygen (pO₂^θ = 1 atm), (h_O − H_O^θ) and (s_O − S_O^θ), respectively. For 2.67 ≤ (3 − δ) ≤ 2.79, (h_O − H_O^θ) decreases with increasing δ, while (s_O − S_O^θ) 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.