An X-ray absorption spectroscopic study on mixed conductive La _0.6Sr_0.4Co_0.8Fe_0.2O _3-δ cathodes. I. Electrical conductivity and electronic structure

The electrical conduction mechanism of mixed conductive perovskite oxides, La_0.6Sr_0.4Co_0.8Fe_0.2O _3-δ, for cathode materials of solid oxide fuel cells has been investigated from electronic structural changes during oxygen vacancy formation. La_0.6Sr_0.4Co_0.8Fe_0.2O _3-δ was annealed under various oxygen partial pressures p(O₂)s at 1073 K and quenched. Iodometric titration indicated that the oxygen nonstoichiometry of La_0.6Sr_0.4Co _0.8Fe_0.2O_3-δ depended on the annealing p(O₂), with more oxygen vacancies introduced at lower than at higher p(O₂)s. X-Ray absorption spectroscopic measurements were performed at the O K-, Co L-, Fe L-, Co K-, and Fe K-edges. The valence states of the Co and Fe ions were investigated by the X-ray absorption near edge structure (XANES) at the Co and Fe L_III-edges. While the Fe average valence was almost constant, the valence of the Co ions decreased with oxygen vacancy introduction. The O K-edge XANES spectra indicated that electrons were injected into the Co 3d/O 2p hybridization state with oxygen vacancy introduction. Both absorption edges at the Co and Fe K-edge XANES shifted towards lower energies with oxygen vacancy introduction. The shift at the Co K-edge resulted from the decrease in the Co average valence and that at the Fe K-edge appeared to be caused by changes in the coordination environment around the Fe ions. The total conductivity of La_0.6Sr_0.4Co_0.8Fe _0.2O_3-δ decreased with decreasing p(O₂), due to a decreasing hole concentration.