Thermodynamic quantities and defect structure of La0.6Sr0.4Co1-yFeyO3-delta(y=0-0.6) from high-temperature coulometric titration experiments

The partial energies and entropies of O-2, in perovskite-type oxides La0.6Sr0.4Co1-yFeyO3-delta (y = 0, 0.1, 0.25, 0.4, 0.6) were determined as a function of nonstoichiometry delta by coulometric titration of oxygen in the temperature range 650-950 degrees C. An absolute reference value of delta was obtained by thermogravimetry in air, The nonstoichiometry at a given oxygen pressure and temperature decreases with. iron content y. At low nonstoichiometries the oxygen chemical potential decreases with delta. The observed behavior can be interpreted by assuming random distribution of oxygen vacancies, an electronic structure with both localized donor states ore Fe, and a partially filled itinerant electron band, of which the density of states at the Fermi level scales with the Co content, The energy of the Fe states is close to the energy at the Fermi level in the conduction band, The observed trends of the thermodynamic quantities can be interpreted in terms of the itinerant electron model only when the iron content is small, At high values of delta the chemical potential of O-2 becomes constant, indicating partial decomposition of the perovskite phase, The maximum value of delta at which the compositions are single-phase increases with temperature, (C) 1997 Academic Press.