Study on Valence Fluctuation by X-ray Anomalous Scattering

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We have developed the X-ray diffraction, scattering and absorption methods to investigate the electronic structures of transition-metal and rare-earth oxides. Anomalous scattering experiments with the X-ray wavelengths close to an absorption edge have made it possible to determine the behavior of ions independently in different valence states (valence-difference contrast method).In the coordination of octahedral oxygen atoms, a chemical shift between Fe^<2+> and Fe^<3+> ions was estimated as 5 eV from the observation of XANES spectra. Based on the absorption data with the algorithm by Cromer and Liberman or Kramers-Kronig transformation, it was found that the anomalous scattering can give a large f' difference between Fe^<2+> and Fe^<3+> ions.The Bragg and diffuse scattering measurements at low temperatures were made with the valence contrast method for topics such as charge ordering and valence fluctuation. The half-indexed reflections were measured for magnetite at 102 K where a charge ordering of Fe^<2+> and Fe^<3+> ions occurs. The low-temperature phase was determined to be Cmcm with a doubled cell of a = 2a_1 + 2a_2, b= -2a_1 + 2a_2, c =2a_3. The X-ray diffuse streaks above the Verwey transition temperature were detected and explained as the result of local charge ordering between Fe^<2+> and Fe^<3+> among the B sites.Radial electron distributions for magnetite were measured to estimate the difference in valence electrons between the A and B sites.