Physique et chimie des matériaux

The aim of this thesis is the investigation of the spectroscopic properties and the electronic structure of vanadium in minerals with a geological and/or technological interest. V K-edge HERFD-XAS experiments were combined with a theoretical determination of the measured spectra. Two approaches were complementary used to calculate the spectra: an ab initio approach and a multielectronic one. Theoretical developments based on a spherical tensor analysis and the coset method were achieved and applied to the X-ray absorption cross-section, in order to extract the spectroscopic properties of the crystal from those of a single site. The spectral signature of V3+ in grossular (Ca3Al2(SiO4)3) was determined using the angular dependence of the XANES pre-edge features. This mineral is therefore a reliable reference compound for further study of the vanadium oxidation state in glasses, minerals and synthetic compounds. Vanadium is also used as a dopant in anatase (TiO2) in which it is incorporated under the +IV oxidation state. The investigation of its local environment showed that vanadium is not incorporated in the Ti-substitutional site but may be incorporated in the interstitial one. Finally, the oxidation state of vanadium in natural and synthetic samples of titanomagnetite (Fe3−xTixO4 :V) from the Bushveld Complex (South Africa) was determined and the +III and +IV oxidation states were shown to coexist. Despite this detailed investigation, the geological implication of their relative variations on the crystallization of the giant magnetite layers is still not well understood.