INTERPRETATION OF PRE-EDGE FINE-STRUCTURES (PEFS) OF K-EDGE OF VANADIUM IN VOPO4, 2H(2)O

A method based on the local density approximation and on the multiple scattering wave calculation taking into account screened X-ray core ho le potential for the final state has been used to analyze the experime ntal XANES spectra of the VOPO4,2H(2)P xerogel. The effect of cluster size on the computations has been analyzed concluding that the pre-edg e depends only on the first shell formed by the six nearest oxygen ato ms. In VOPO4,2H(2)O structure, the absorbing VO6 octahedron is surroun ded by PO4 tetrahedra. Thus VO6 octahedra are not linked with each oth er excluding solid state effect in the pre-edge region. In contrast, e dge region is sensitive to the cluster size. Computations show that at least a cluster of V5P4O26 must be taken into account to reproduce, i n rather good agreement, the edge part of the absorption spectrum.