Xanes studies on rhenium L absorption edges of Re2O7 graphite intercalation compounds and of other rhenium-oxygen compounds

The electronic structure as well as the medium-range order of rhenium oxides and Re2O7 graphite intercalation compounds (GICs) were investigated by analyzing the XANES (X-ray absorption near edge structure) of the Re L(I-III) absorption edges. The Re L(II,III) edges show for all perrhenates strong white lines whose full widths decrease going from the solid to the highly concentrated to the highly diluted solution. Coincidently the area of the white lines increases. The GICs show the same dependence with the exception that their absorption peaks are always lower, which is caused by a charge transfer from graphite to rhenium. A reduced intensity of the strong pre-edge peak at the Re L(I) edges in comparison to solid perrhenates and perrhenate solutions indicates a slightly distorted tetrahedral symmetry for the GICs. The analysis of a multiple scattering feature at the Re L(I) edge reveals for the GIC intensities similar to the solutions but several times lower than those of the solid perrhenates. The loss of the three-dimensional long-range order after the intercalation accompanied by a low density of ReO4 tetrahedra causes this effect.