Valence and Coordination State Analysis for Transition Metal Ions Mixed Conductive Perovskite Oxides

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Rare earth containing perovskite oxides with mixed conductive properties in which both ions and electrons move as charge carriers were heat-treated under various oxygen partial pressures and temperatures for various kinds of alkaline earth compositions. The crystal structures were precisely analyzed by powder X-ray Rietveld analysis. Then the relations between the changes in valence and coordination states of the transition metal ions and the properties such as electrical conductivity etc.were investigated. La : Mn=1 : 1, La substituted La-Sr-Mn-O or Mn substitute La-Mn-Mg-O perovskite oxides were heat-treated under various oxygen partial pressures. The Mn valence of the samples were measured by a chemical analysis, and the crystal structure were refined by the powder X-ray diffraction and the Rietveld analysis. A novel orthor-hombic perovskite phase containing Mn^<2+> ions was formed by heat-treatment under low oxygen partial pressures. The structure was very close to a cubic symmetry. It is supposed that the micro Jahn-Teller effects of Mn^<2+> ions were nearly canceled by one another. Crystal structure of perovskite La_<1-x>Sr_xCoO_3 was precisely determined by powder X-ray Rietveld analysis, the mean valence of cobalt ion was measured by iodometry, and the correlation with the electrical property was investigated. The valence of the cobalt ion showed a stoichiometric change for the lower amount of Sr, however, the deviation from the stoichiometry increased with the increase of Sr. With the increase of Sr amount, there observed an abrupt decrease for CoO distance by the Rietveld analysis, and the transition from semiconducting to metallic in the electrical conductivity. It is considered that the charge transfer gap existing between O-2p band and Co-d band was closed by the change in the crystal structure.