High-Energy X-ray Absorption Spectroscopy: A New Tool for Structural Investigations of Lanthanoids and Third-Row Transition Elements

This is the first systematic study exploring the potential of high-energy EXAFS as a structural tool for lanthanoids and third-row transition elements. The K-edge X-ray absorption spectra of the hydrated lanthanoid(III) ions both in aqueous solution and in solid trifluoromethanesulfonate salts have been studied. The K-edges of lanthanoids cover the energy range from 38 (La) to 65 keV (Lu), while the corresponding energy range for the L,edges is 5.5 (La) to 9.2 keV (Lu). We show that the large widths of the core-hole states do not appreciably reduce the potential structural information in the high-energy K-edge EXAFS data. Moreover, for lanthanoid compounds, more accurate structural parameters are obtained from analysis of K-edge than from L-3-edge EXAFS data. The main reasons are the much wider k range available and the absence of double-electron transitions, especially for the lighter lanthanoids. A comparative K- and L-3-edge EXAFS data analysis of nonahydrated crystalline neodymium(III) trifluoromethanesulfonate demonstrates the clear advantages of K-edge analysis over conventionally performed studies at the L-3-absorption edge for structural investigations of lanthanoid and third-row transition metal compounds. The coordination chemistry of the hydrated lanthanoid(III) ions in aqueous solution and solid trifluoromethanesulfonate salts, based on the results of both the K- and L-3-edge EXAFS data, is thoroughly discussed in the next paper in this series