POLARIZED XANES AND EXAFS AT THE V-K-EDGE OF VOPO4.2H2O GEL, COMPARISON WITH THE V-K-EDGE IN V2O5 XEROGEL

Coatings have been obtained by deposition of colloidal solutions of VOPO4 and V2O5 onto a flat substrate. The strong pre-edge anisotropy of these materials reveals a good texturation and organization of these gels even at the molecular scale. Using EXAFS on V2O5 single crystal and VOPO4 layers, we deduce first that the short axial bond lengths are the same in both compounds, and second that pre-edge can be used even on powders to track any change in axial bonds. Comparing polarized XANES and crystallographic structures, we show that it is a sensitive probe, allowing one to detect weak asymmetries in the ligand coordination sphere. However, the lack of theoretical basis of this effect prevents any quantitative estimate.