Polarization dependent x-ray-absorption measurements were performed on pure and Cr-doped V 2 O 3 single crystals in the antiferromagnetic insulating, paramagnetic insulating, and metallic phases. The orbital occupation of the V 3d 2 ion is found to depend appreciably on the phase, but always with the Sϭ1 character, requiring an explanation which is beyond the elegant simplicity of the pure one-band Hubbard model or of models in which the a 1g orbital is projected out by means of a simple dimerization. The results reveal the critical role of the spin and orbital dependence of the on-site 3d-3d Coulomb energy, and a mechanism is proposed to explain the closing or opening of the band gaps which are of much higher energy scale than the transition temperatures.

We report combined experimental and theoretical investigations of x-ray absorption at the Ru-L 2,3 and O-K thresholds of the Ru͑IV͒ compounds RuO 2 and Sr 2 RuO 4 and of the Ru͑V͒ compound Sr 4 Ru 2 O 9 . Significant differences in the intensity distribution of the t 2g -related and e g -related peaks between the L 3 and the L 2 edges are found, due to the combined effects of 4d spin-orbit coupling and the interelectronic Coulomb interaction described by the Slater integrals. The observed spectral features can be well reproduced by crystal-fieldmultiplet calculations. With increasing the Ru valence from IV to V, the spectra are shifted by Х1.5 eV to higher energy at the Ru-L 2,3 edges and Х1.0 eV to lower energy at the O-K edge, which is of the same order of magnitude as on going from the divalent to the trivalent late 3d transition-metal oxides.

We report experimental and theoretical results on the Fe K edge x-ray absorption spectrum and 1s2 p resonant inelastic x-ray scattering ͑RIXS͒ spectra in ␣-Fe 2 O 3 . The results are interpreted using an FeO 6 9Ϫ cluster model with intra-atomic multiplet coupling and interatomic covalency hybridization. The 1s2 p RIXS is treated as a coherent second-order optical process. It is shown that the double-peak structure in the pre-edge region of Fe K absorption spectrum is due to the cubic crystal-field splitting, and that the intensity of the e g (t 2g ) component in the 1s2p resonant inelastic spectrum is enhanced by tuning the incident photon energy to the e g (t 2g ) component in the absorption spectrum. ͓S0163-1829͑98͒08443-4͔

The reduction behavior of Co/TiO 2 and Co/Mn/TiO 2 catalysts for Fischer-Tropsch synthesis has been investigated by soft X-ray absorption spectroscopy (XAS). In situ XAS measurements of the L 2,3 edges of Co and Mn have been carried out during reduction treatments of the samples in H 2 at a pressure of 2 mbar and at temperatures up to 425°C. The changes of Co and Mn 3d valences and the symmetries throughout the reduction have been determined by comparison with theoretical calculations based on the charge transfer multiplet code. Furthermore, bulk Co 3 O 4 has been reduced under the same conditions to evaluate the effect of TiO 2 as a support on the reducibility of Co oxides. The average Co valence at the various temperatures has been determined from a linear combination of the reference spectra. It was found that the unsupported Co 3 O 4 was easily reduced to Co 0 at 425°C, whereas the Co 3 O 4 supported on TiO 2 catalysts was only reduced to a mixture of CoO and Co 0 , even after 12 h reduction at 425°C. The presence of Mn further retards the reduction of the supported Co 3 O 4 particles. The Mn III ions were easily reduced to MnO at temperatures lower than 300°C , and they remained in this oxidation state even after further temperature increase. In addition, catalytic tests in the Fischer-Tropsch synthesis reaction at a pressure of 1 bar indicate that the selectivity of these catalysts might be related to the extent of Co reduced after the activation treatment (i.e., the reduction with H 2 ).

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