Electronic States and Physical Propertied of Mixed Valent Metal Oxides

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Transition metal ions in their oxides show various valence states which cause variety of physcal and clemical properties. however, the valence states of the transition metal ions have not well been understood, because of their complexity, namely, d orbital levels, interatomic interactions due to overlap between metal d and oxygen p orbitals.In the present research, we have calculated the electronic structures of cluster models for the transition metal oxides by the use of the DV-X alpha method, and have investigated valence states and chemical bondings of the transition metal lolls. It is noted that mixed valent states tend to appear in the oxides of high-valent ions such as Fe^<4+> and Cu^<3+> which possess interesting physical properties. At low temperatures, disproportionation reaction 2Fe^<4+> -> Fe^<3+> + Fe^<5+> in a perovskite type Fe^<4+> oxide has been reported. Among Cu^<3+> oxides, high-temperature superconductivity has been found. The results of the cluster model calculations for these oxides show good agreement with various experimental results. For Fe^<4+> oxide, the calculation can well explain the experimental results of Mossbauer effect and the model that the disproportionation reaction is caused by quenching of lattice vibration at low temperature has been proved. For Cu^<3+> oxide, the results of calculations are in good agreement with the experimental photoemission spectra and can interpret the magnetic properties of high-temperature superconductors. For both these oxides, a large amount of charge transfer from oxygen to metal is one of the characteristics of the electronic state.