Influence of multi-electron excitation on EXAFS spectroscopy of trivalent rare-earth ions and elucidation of change in hydration number through the series

Abstract

We have made a detailed study of the extended X-ray absorption fine spectra (EXAFS) at the K edge of aqueous Y ion and at L 3 edges of aqueous lanthanide ions and thereby elucidated the systematic changes in their hydration structures. Anomalous peaks arising from double-electron excitation (2p, 4d → 5d, 5d) appear in the EXAFS signals of La 3+ -Tb 3+ between 5-7 Å−1. We established a removal process of double-electron excitation from EXAFS spectra. Using that process, we confirmed that the intensity and energy position of the extracted double-electron excitation are comparable to previously reported data. The presence of double-electron excitation engenders a smaller error than the errors estimated in the fitting process. Consequently, double-electron excitation does not seriously affect the determination of the structures of REE 3+ aquo ions in the first coordination sphere. Subsequent EXAFS analyses of hydrated REE 3+ ions suggest that the hydration numbers, the interatomic distances, and the Debye-Waller factors decrease from 9.7, 2.55 Å, and 9.0 × 10 −3 Å 2 for La 3+ to 7.9, 2.31 Å, and 5.7 × 10 −3 Å 2 for Lu 3+ . These parameters change as a sigmoid curve with increasing atomic number. The hydration structures of REE 3+ ions are inferred to change from the nonahydrated structure for La 3+ -Nd 3+ to the octahydrated structure for Tb 3+ -Lu 3+ through intermediate structures for Sm 3+ , Eu 3+ , and Gd 3+ . In addition, the hydration state of Y 3+ closely resembles that of Ho 3+ because the two have almost identical ionic radii.