CeO2 X-RAY RESONANT SPECTROSCOPY
نویسندگان
چکیده
Various samples of CeO prepared under different experimental conditions have been studied by X-ray pho$oabsorption comparatively to Ce metal. For substoichiometric samples, peaks characteristic of the ce3+ ion are predominant. For"purel' dioxyde the spectra reveal no localized 4f states are present in the ground state while extended states with 4f character are observed confirming partial covalency of bonding in CeO 2 ' INTRODUCTION The electronic structure of cerium in its various compounds is still a subject of controversy. In insulating compounds such as CeOg, with the chemists' view, Ce is tetravalent and no localized 4f states are present. On the other hand, on the basis of the mixed-valence point of view, a valence almost equal to three has been suggested for Ce in any chemical environment, corresponding to a ground state of 4f 1 configuration; this last argument was based on results obtained from XPS and L3 photoabsorption spectroscopies. Indeed several experimental results, in particular from optical ( 1 ) and bremsstrahlung isochromat (2) spectroscopies show the absence of localized 4f electron in Ce02 . Among the techniques in which a core hole is present at the final state, the nd photoabsorption spectroscopy is the most convenieht to determine the 4f configuration for two reasons: first, the coupling between the 3d hole and the unpaired 4f electrons dominates the fine structure which is thus sensitive to the number of 4f electrons. Second, no final state effect is present because the core hole is screened by the 4f excited electron, making these photoabsorption spectra we13 interpreted in terms of initial state effects. On the other hand, Ehe 3d spectra are simpler than the 4d ones because the spin-orbit coupling is larger than the d-f interaction energy and multiplet structures are located under their own ionization threshold makfng autoionization negligible. With this in mind, we have investigated the Ce3d absorption and emission spectra of Ce metal and Ce02 in the aim to determine which spectral features allow to Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19879183 C9-1022 JOURNAL DE PHYSIQUE identify the 4f configuration in the Ce compounds.
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