Electronic structure of hydrated Ce4+ ions in solution: An x-ray-absorption study.

Abstract

X-ray-absorption measurements at the Ce ${L}_{3}$ and ${L}_{2}$ edges have been performed for a series of acidic solutions of formally ${\mathrm{Ce}}^{4+}$(4${f}^{0}$) hydrated ions at various concentrations. It is found that the ${L}_{3}$,2-edge x-ray-absorption near-edge structure of these ions exhibit two white lines. This apparent double-white-line feature is observed for the first time for formally ${\mathrm{Ce}}^{4+}$(4${\mathrm{f}}^{0}$) centers (hydrated ions) in liquid solutions. Except for a smaller energy separation, the double-white-line feature resembles the well-known ${L}_{3}$-edge characteristics of many Ce and rare-earth valence-fluctuating (mixed-valence) metallic compounds and the more recently reinterpreted interatomic-intermediate-valence compounds, ${\mathrm{CeO}}_{2}$ and Ce(SO${)}_{4}$\ensuremath{\cdot}4${\mathrm{H}}_{2}$O, which are insulators in the solid state. These results are discussed in terms of the initial-state-configuration interaction of the localized, ionic 4${f}^{0}$ and 4${f}^{1}$L states (L denotes a hole in the ligand). This model has been recently proposed by Bianconi et al. to explain the apparent double-white-line feature observed in ${\mathrm{CeO}}_{2}$ and similar nonmetallic rare-earth systems. The ${L}_{3}$-edge white-line profiles of Ce(IV) compounds and complexes are correlated with ionicity. Analysis of the extended x-ray-absorption fine structure of the ${\mathrm{Ce}}^{3+}$ and ${\mathrm{Ce}}^{4+}$ hydrated ions yields a more accurate Ce-${\mathrm{H}}_{2}$O bond-length difference of 0.080(5) A\r{} between the two different oxidation states in solution. Implication of the results to the [Ce(${\mathrm{H}}_{2}$O${)}_{\mathrm{n}}^{3+}$]/[Ce (${\mathrm{H}}_{2}$O${)}_{\mathrm{n}}^{4+}$] (n\ensuremath{\approxeq}9) electron exchange reaction is also discussed.