A thorough study of Mn K-edge x-ray-absorption near-edge spectroscopy ͑XANES͒ on several manganites has been performed. The spectra of La 1Ϫx Ca x MnO 3 have a similar shape for the whole series. The chemical shift for intermediate compositions lies between the two end members of the series showing an intermediate oxidation state of the Mn atom. The analysis of the spectra indicates that the Mn atom does not fluctuate between 3ϩ and 4ϩ states. We have also collected XANES spectra of magnetoresistive perovskites (x Ϸ0.33) at different temperatures and at different external magnetic fields. Very small differences in the XANES spectrum are induced by decreasing the temperature or by applying a magnetic field showing that the electronic state of the Mn atom across the metal-insulator transition remains essentially unaltered. X-ray circular magnetic dichroism at the Mn K edge in the ferromagnetic phase also shows a unique magnetic signal.

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Resonant x-ray scattering was used to investigate electronic fluctuations of the octahedral iron atoms in magnetite. We measured the (002) and (006) "forbidden" x-ray diffraction reflections permitted by the anisotropy of the iron anomalous scattering factor. The energy and azimuthal angle dependencies of these reflections, and the polarization analysis, are shown and discussed. The results clearly show p and d iron empty states ordering in magnetite at room temperature. Moreover, the octahedral iron atoms are electronically equivalent in a time scale lower than 10(-16) sec. Therefore, magnetite should be considered as an itinerant magnet and not as a fluctuating mixed valence material.

We reconsider the interpretation of x-ray resonant scattering experiments performed on the so-called charge-ordered manganites with Mn3+/Mn4+ = 1. The comparison between these experiments and our x-ray absorption data shows that no real Mn3+/Mn4+ charge ordering occurs in these compounds. However, these experiments demonstrate the presence of two different types of manganese atom with a different local geometrical structure. We propose a structural model which accounts for these resonant scattering experiments. Within this model, the charge-ordering phase transition can be explained as a structural phase transition driven by the softening of a phonon mode. This produces a periodic arrangement of local distortions, which is responsible for the observed resonances. Furthermore, electronic localization in these materials would occur on a length scale larger than the atomic one.

Extended x-ray absorption fine structure at the manganese K edge has been used to study the first oxygen coordination shell at the manganese site as a function of temperature in mixed-valence perovskites. La 1Ϫx Ca x MnO 3 ͑xϭ0, 0.15, 0.33, 0.5, 0.66, and 1͒, La 0.60 Y 0.07 Ca 1/3 MnO 3 , La 0.57 Tb 0.1 Ca 1/3 MnO 3 , and Tb 1Ϫx Ca x MnO 3 ͑xϭ0, 0.33, 0.5, and 0.67͒ were studied. Our results show that the manganese atom is surrounded by a regular oxygen octahedra in the metallic-ferromagnetic phase. The analysis of the spectra at temperatures above the magnetic transition can be described in terms of either a unique Mn-O interatomic distance with a large Debye-Waller factor or two Mn-O distances with a distortion lower than 0.1 Å. Such distortion or disorder must be dynamic since it cannot be detected by diffraction measurements. This result supports the evidence of a large phonon-electronic interaction and the polaronic origin as a possible mechanism of electrical conduction in the paramagnetic phase. ͓S0163-1829͑98͒03101-4͔

We present a detailed analysis of x-ray absorption near-edge structure spectra ͑XANES͒ of transition-metal aqueous complex in solution. The metal K edges of Ni 2ϩ and Co 2ϩ are analyzed by multiple-scattering theory using clusters of different size and different final-state potentials. On this basis, the effect of hydrogen atoms of the water molecules on the theoretical XANES spectra has been investigated. The calculations show that the intensity ratio and width of the main features of the spectrum are sensitive to the presence and the geometrical position of hydrogen atoms. A very good agreement between experimental data and theoretical calculations has been obtained using final-state potentials built on the basis of the self-consistent field procedure and clusters with a well-defined geometrical position of hydrogen atoms.

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