Structural properties of actinides probed by high energy resolution X-ray spectroscopy

The actinide (An) M4,5 edge high energy resolution X-ray absorption near edge structure (HR-XANES) and core-to-core 3d4f resonant inelastic X-ray scattering (3d4f RIXS) are becoming increasingly important for electronic structure and speciation studies of actinide materials [1–6]. The potential of these techniques to reveal bonding properties – reactivity relations of uranium and neptunium will be demonstrated. The experiments were performed at the CAT-ACT beamline at the KIT light source, which operates the Karlsruhe Research Accelerator (KARA), Karlsruhe Germany [4].
A. One key question in actinide sciences is how the bond covalency, well documented for the uranyl(V)/(VI) (U(V)O2 1+ or U(VI)O2 2+), relates to the complex stability. The electronic structure and bonding properties of U(IV), U(V) and U(VI) coordination complexes with similar structures but major reactivity differences will be compared. Using a combination of U M4,5 HR-XANES spectroscopy techniques and ab-initio multireference CASSCF computations, it will be provided evidence for the stabilization of U(V)-yl compounds when the bond covalency of the U(V)-Oax bond is reduced by Fe(II) or when there is lack of covalency in the bonding between the U(V)-yl and the equatorial ligands [6].
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B. We precipitated Np(V) and U(VI) from an aqueous potassium−sodium-containing carbonate-rich solution, and investigated the solid phases. It will be shown that despite the 100 times lower initial Np(V) concentration at pH 10.5 and oxic conditions, Np(V)-rich phases predominately precipitate. The prevailing formation of Np(V) over U(VI) solids demonstrates the high structural stability of Np(V) carbonates containing potassium. It will be illustrated that the Np M5 edge HR-XANES spectra are sensitive to changes of the Np−O axial bond length for Np-yl-(V/VI). U/Np M4,5 edge HR-XANES spectroscopy and Np 3d4f RIXS are applied in combination with thermodynamic calculations, U/Np L3 edge XANES, and extended X-ray absorption fine structure (EXAFS) studies to analyse the local atomic coordination and oxidation states of uranium and neptunium. The XANES/ HR-XANES analyses are supported by ab initio quantum-chemical computations with the finite difference method near-edge structure code (FDMNES) [2].
[1] a) Vitova T, et al., Nat Commun 8, 16053 (2017); b) Pidchenko I, et al., Environ. Sci. Technol. 51, 2217 (2017). [2] Vitova T, et al. Inorg Chem 59, 8 (2020). [3] Zegke M, et al., Chem Sci 10, 9740 (2019). [4] Zimina A, et al., Rev Sci Instrum 88, 113113 (2017). [5] Kauric G, et al., Materials Today Advances 8, 100105 (2020). [6] T. Vitova, et al., submitted