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

The oxygen nonstoichiometry delta of La1-xSrxCo1-yFeyO3-delta (x=0.6 and y=0.2, 0.4) was investigated by thermogravimetry in the range 703 <= T/degrees C <= 903 and 1E-5 < pO(2)/atm < 1. The oxygen deficit increases with increasing T and decreasing pO(2). Electronic conductivities sigma were measured as a function of pO(2) in the range 1E-5 < pO(2)/atm < 1 at 700 <= 5 T/degrees C <= 900. At constant T, a p-type pO(2)-dependence of sigma is observed. Oxygen nonstoichiometry data are analyzed with regard to the enthalpy and entropy of oxidation Delta H-ox(theta) and Delta S-ox(theta), as well as to the partial molar enthalpy and entropy of oxygen with respect to the standard state of oxygen (pO(2)(theta) = 1 atm), (h(O) - H-O(theta)) and (s(O) - S-O(theta), respectively. For 2.67 <= (3-delta) <= 2.79, (h(O) - H-O(theta)) decreases with increasing delta, while (s(O) - S-O(theta)) 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 a on delta is modelled, using calculated defect concentrations as functions of delta. Deviations from the ideal behaviour suggest an immobilization of n-type charge carriers by oxygen vacancies. (c) 2006 Elsevier B.V. All rights reserved.