Electronic structure of hydratedCe4+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${)}_4$⋅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) Å 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≊9) electron exchange reaction is also discussed.