Bonding properties of uranium carbonate species in liquid and solid state in multiple oxidation states studies by U M4-edge high resolution X-ray spectroscopy

It is a long-standing discussion how the 5f electrons of the actinide (An) elements participate in chemical bonds. For investigating this and specifically the covalency of the An-ligand-bond [1, 2, 3], X-ray emission spectroscopy (XES) techniques, such as high-resolution x-ray absorption near edge structure (HR-XANES) [4] and valence band resonant inelastic x-ray scattering (VB-RIXS) [5], are advanced tools. In this study the changes in the electronic structure were investigated for uranium carbonate complexes with the oxidation states from IV to VI in solid and - for the first-time using the VB-RIXS technique – in liquid phase. A spectro-electrochemical cell for simultaneous measurement of XES and UV-Vis spectroscopy while applying a potential was successfully used to change in-situ the U oxidation state from VI to V while suppressing the disproportionation of U(V) [6].
U M4 edge HR-XANES and VB-RIXS experiments were performed at the ACT station of the CAT-ACT beamline [7, 8]. The U coordination environment in solution was confirmed with U L3-edge EXAFS at the INE-Beamline [9]; both beamlines are at the KIT Light Source, KIT, Germany [10]. ... mehrA solution with 34 mM U in 1.5 M sodium carbonate at pH 12.5 was used to measure U M4 edge HR-XANES and VB-RIXS for [U(VI)O2(CO3)3]4- species and after reduction to [U(V)O2(CO3)3]5- carbonate species. A solution sample of 31.7 mM [U(IV)(CO3)5]6- at pH 8.3 as well as solid samples of Na4[U(VI)O2(CO3)3] and Ca[U(VI)O2(CO3)3x10H2O] were studied too.
A decrease in covalency of the O=U=O axial bond is detected using the HR-XANES spectra when reducing U(VI)-yl to U(V)-yl and U(V)-yl to U(IV). With a decreasing oxidation state of the uranium atom, the U 5f and 6p electron population of the occupied valence molecular orbitals also decreases as illustrated by the VB-RIXS spectra. This result suggests reduction of covalency for both axial and equatorial bonds. We will demonstrate that the U M4 edge VB-RIXS technique is valuable for investigations of U 5f and 6p electron contributions in the occupied valence band of [U(VI)O2(CO3)3]4- in solution and Na4[U(VI)O2(CO3)3], Ca[U(VI)O2(CO3)3x10H2O] solids.
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