Preliminary Evaluation of Local Structure and Speciation of Lanthanoids in Aqueous Solution, Iron Hydroxide,...

We elucidate the application of L₃-edge X-ray absorption near edge structure (XANES) spectra to the local structural analysis of lanthanoids in aqueous solution, iron hydroxide, manganese dioxide, and calcium carbonate. The L₃-edge XANES spectra of lanthanoid compounds showed sharp white lines. The full width at half-maximum (FWHM) values of lanthanoid aqua ions exhibited a convex tetrad curve in the series variation across the lanthanoid series. The variation is attributable to 4f electron orbitals and can be explained by the refined spin-pairing energy theory. For each lanthanoid, the FWHM values of lanthanoid compounds roughly decreased with increasing local coordination numbers. However, they did not faithfully reflect the local coordination sphere of the lanthanoid complex having a high and distorted coordination sphere and were rather sensitive to their chemical forms. The relationship between the magnitude of the FWHM values was determined by the crystal field splitting or degeneracy of 5d orbitals. The systematic variation of FWHM can be explained by the ligand strength of the ligand molecules (-H₂O⁰, -O^-, -OH^-, -CO₃^2-, -Cl^-, and -O^2-) that cause the crystal field splitting. Therefore, the FWHM values of L₃-edge XANES of lanthanoid compounds may be more useful in speciation analysis rather than structural analysis.