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Unique sensitivity to actinide bond covalency revealed

24-10-2024

Resonant inelastic X-ray scattering (RIXS) is a powerful technique for probing the covalency of actinide-ligand bonds. This study demonstrates how RIXS provides detailed insights into uranium 5f orbital interactions, advancing the understanding of actinide bonding – an important step towards improving nuclear waste management and sustainable energy solutions.

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Advances in nuclear waste management and the development of new nuclear power strategies depend on a detailed understanding of actinide chemical interactions. This understanding is critical for reducing costs and risks of decommissioning legacy nuclear sites and achieving net-zero emissions [1].

A key aspect of these interactions is actinide-ligand bond covalency – the sharing of electron density between atoms in chemical bonds. The involvement of actinide 5f orbitals in bonding significantly influences the physical and chemical properties of actinide compounds. However, quantifying the impact of bonding interactions on these orbitals has remained a challenge [2].

The concept of central-field covalency describes how bonding interactions expand the 5f radial distribution, thereby weakening interelectron repulsions [3]. Yet, only a few experimental techniques are sensitive enough to measure these effects. 

This study shows that RIXS is highly sensitive to the central-field expansion of uranium 5f orbitals, focusing on three molecular complexes: [UF6]2-, [UCl6]2-, and [UBr6]2-. Using the Tender Energy X-ray Spectrometer (TEXS) at the ID26 beamline, the M4,5 edges of uranium were measured with high sensitivity and energy resolution.

In the RIXS process, a core 3d electron is excited into an unoccupied 5f state (3d->5f), followed by the relaxation of a core 4f electron (4f ->3d). This technique enhances spectral resolution and provides detailed insights into 4f-5f interelectron repulsion and exchange interactions. 

RIXS data can be analyzed by slicing the RIXS planes into 1D cuts, each offering specific information (Figure 1). High-energy resolution fluorescence detection (HERFD) slices provide details on unoccupied 5f orbitals, while resonant X-ray emission spectroscopy (RXES) cuts reveal the interactions between the 4f and 5f electron shells.
 

Fig1.jpg

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Fig. 1: a) 3d4f RIXS map at the M4-edge for [UCl6]2-, showing HERFD and RXES slices through the maximum intensity features. b) Stack plot of the HERFD, highlighting satellites related to 5f interelectron repulsion. c) Overlay of RXES, featuring satellites associated with 4f-5f spin exchange. d) As the halide changes from F to Cl to Br, shifts in energy and intensity of the satellites indicate reduced interelectron repulsion and spin-exchange, corresponding to increased 5f orbital covalency. 


Distinct satellite features observed in the RIXS spectra showed variations in intensity and energy depending on the bonding halide ligand atom (Figure 1b-c). By combining density functional theory (DFT) and ligand field multiplet theory, these features were attributed to 5f interelectron repulsion (in HERFD satellites), and 4f-5f spin-exchange (in RXES satellites).

The experimental data were accurately simulated by adjusting the energies of electronic repulsion and exchange, revealing central-field covalency trends across the compounds. Both 5f interelectron repulsion and 4f-5f spin-exchange decreased as the halide ligand changed from fluorine to chlorine to bromine, indicating increased uranium-ligand 5f covalency. 

Although DFT accurately predicts 5f interelectron repulsion, it overestimates 4f-5f spin-exchange. The discrepancy arises from errors in calculating radial distribution functions, which represents the variation in the density of actinide 4f and 5f orbitals with radial distance (Figure 2a).
 

Fig2.jpg

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Fig. 2: a) Radial distribution functions (RDFs) of the 5f orbitals for the three compounds, compared to the 4f and 5f RDFs of a free U(IV) ion. The 4f-5f overlap, and consequently spin-exchange, decreases from [UF6]2- to [UCl6]2- to [UBr6]2-, as 5f central-field covalency increases. b) Ligand field multiplet simulation of [UCl6]2- RXES, showing a significant reduction in the calculated 4f-5f spin exchange is required to match the experimental data.


It has been demonstrated that 4f-5f spin-exchange depends on the relative phase and overlap of these radial distribution functions, with the central-field expansion effect only applying to 5f orbitals. To align theoretical predictions with experimental results, a significant 5f central-field expansion was needed (Figure 2b), confirming that 4f-5f spin-exchange is highly sensitive to 5f orbital expansion.

This work establishes 3d4f RIXS as a highly sensitive probe for studying 5f central-field covalency, demonstrating its value in actinide chemistry. Unlike other methods used to quantify 5f covalency, RIXS can be applied across all actinide elements, regardless of oxidation state, coordination geometry, or ligand type. The findings underscore the importance of experimental validation of theoretical models, especially in cases where DFT calculations may fall short due to relativistic approximations that underestimate 5f orbital expansion. 

Overall, this research opens new opportunities for advancing the understanding of actinide bonding, with broad applications in analytical chemistry and materials characterization. By providing more precise insights into 5f covalency, these findings could contribute to advancements in nuclear waste management and the development of sustainable nuclear technologies.



Principal publication and authors
Determination of Uranium Central-Field Covalency with 3d4f Resonant Inelastic X-ray Scattering, T.G. Burrow (a), N.M. Alcock (a), M.S. Huzan (a), M.A. Dunstan (b), J.A. Seed (a), B. Detlefs (c), P. Glatzel (c), M.O. Hunault (d), J. Bendix (e), K.S. Pedersen (b), M.L. Baker (a), J. Am. Chem. Soc. 146, 32, 22570-22582 (2024); https://doi.org/10.1021/jacs.4c06869
(a) University of Manchester, Manchester (UK)  
(b) Technical University of Denmark, Kongens Lyngby (Denmark)
(c) ESRF
(d) Synchrotron SOLEIL, L’Orme des Merisiers, Saint-Auban (France)  
(e) University of Copenhagen, Copenhagen (Denmark)


References
[1] N. Kaltsoyannis & S.T. Liddle, Chem 1, 659-662 (2016).
[2] M.L. Neidig, D.L. Clark & R.L. Martin, Coord. Chem. Rev. 257, 394-406 (2013).
[3] C.K. Jørgensen, Orbitals in Atoms and Molecules (Academic Press: London), (1962).

 

About the beamline: ID26

ID26 is dedicated to X-ray absorption and emission spectroscopy of complex systems in the tender and hard X-ray range. The high-brilliance X-ray beam allows for spectroscopic studies of samples with low analyte concentration and challenging matrices. X-ray emission spectroscopy is performed by means of crystal analyzer spectrometers. By combining the tuneable incident energy with an emission spectrometer, it is possible to take advantage of resonance effects that can provide detailed information on the electronic structure.

The local coordination and electronic structure of an X-ray absorbing atom are studied by extended X-ray absorption fine structure (EXAFS), X-ray absorption near edge structure (XANES), X-ray emission (XE) and resonant inelastic X-ray scattering (RIXS) spectroscopy. The techniques probe occupied and unoccupied electron orbitals, providing a wealth of information. It is thus possible to study orbital splittings, spin- and oxidation states as well as the coordination symmetry and ligand type. RIXS gives access to element-specific excitations of only a few eV that can arise from local (e.g., d-d), nearest neighbour (e.g., charge transfer) and collective excitations.

With the tender and hard X-ray probe, very few restrictions apply to the sample environment. ID26 can host cryostats and cells for in-situ and operando studies to carry out experiments in applied sciences including coordination chemistry, (bio)catalysis, materials science, electro-chemistry and environmental sciences.