Polarization Dependent High Energy Resolution X-ray Absorption Study of Dicesium Uranyl Tetrachloride

Abstract

Dicesium uranyl tetrachloride (Cs2UO2Cl4) has been a model compound for experimental and theoretical studies of electronic structure of U(VI) in the form of UO2(2+) (uranyl ion) for decades. We have obtained angle-resolved electronic structure information for oriented Cs2UO2Cl4 crystal, specifically relative energies of 5f and 6d valence orbitals probed with extraordinary energy resolution by polarization dependent high energy resolution X-ray absorption near edge structure (PD-HR-XANES) and compare these with predictions from quantum chemical Amsterdam density functional theory (ADF) and ab initio real space multiple-scattering Green's function based FEFF codes. The obtained results have fundamental value but also demonstrate an experimental approach, which offers great potential to benchmark and drive improvement in theoretical calculations of electronic structures of actinide elements.

The role of the 5f valence orbitals of early actinides in chemical bonding

“…. U(VI) does not have 5 f electrons in the ground state, and we showed previously that the spectrum can be well-described with ground state DFT electronic structure calculations17; the deviations between calculated and measured relative energies are <0.6 eV. Hay et al .…”

“…For simplicity, we use only the assignments σ, π, δ and ϕ, for the D ∞h symmetry of linear AnO 2   2+ . An M 4,5 absorption edge HR-XANES and 3 d 4 f RIXS directly measure the relative energies of the unoccupied 5 f δ/ϕ, 5 f π* and 5 f σ* orbitals of the An in the same spectrum, thus avoiding experimental artifacts introducing uncertainties due to different sample preparation protocols or variable measurement conditions17. Especially, the 5 f σ* orbital can be evaluated with high precision.…”

“…3d). We have previously been able to assign these to transitions of 3 d electrons to 5 f δ/ϕ, 5 f π* and 5 f σ* orbitals, respectively17. The first two peaks 5 f δ/ϕ and 5 f π* are not resolved in the NpO 2   2+ and PuO 2   2+ RIXS maps/HR-XANES, but a peak shoulder (indicated with an arrow and 5 f π*) and asymmetry are evident for NpO 2   2+ (Fig.…”

“…We are applying An M 4,5 absorption edge high-energy resolution XANES (HR-XANES; XANES=X-ray absorption near edge structure) and 3 d 4 f resonant inelastic X-ray scattering (3 d 4 f RIXS) as bulk sensitive techniques, which can be applied in-situ and under extreme conditions (for example, high/low temperatures or pressures) for investigations of matter in any state13141516171819. Particularly 3 d 4 f RIXS is a novel experimental tool for comparison of the level of localization of unoccupied 5 f orbitals across the An series and their participation in chemical bonding.…”

Differential uranyl(v) oxo-group bonding between the uranium and metal cations from groups 1, 2, 4, and 12; a high energy resolution X-ray absorption, computational, and synthetic study

“…Experimentally, High Energy Resolution X-ray Absorption Near Edge Structure (HR-XANES) and Resonant Inelastic X-ray Scattering (RIXS) have recently emerged as very sensitive tools for characterisation of oxidation states and electronic structures of actinide elements. [28][29][30][31][32][33][34][35][36][37][38] In particular, since they directly probe the An 5f valence unoccupied orbitals, they can distinguish small variations in the chemical bonding at the metal for uranyl(VI) and uranyl(V) compounds. These experimental spectroscopic techniques are most readily interpreted in combination with computational analyses.…”