Quantifying Actinide-Ligand Covalency with Resonant Inelastic X-ray Scattering

The UK has invested in world-leading synchrotron X-ray facilities. This includes state-of-the-art resonant inelastic X-ray scattering (RIXS) techniques that measure electronic structure with unprecedented energy resolution. RIXS is, however, infrequently applied to its full potential due to a lack of analytical expertise that limits quantitative insights. The least developed area of all is the analysis of actinide RIXS. Actinides are amongst the largest elements within the periodic table, and their chemistry is notoriously challenging to predict. The development of RIXS at energies suitable for accessing actinides has opened up an unexplored avenue to experimentally measure actinide physical and chemical properties. RIXS is an element-specific technique that targets the outer actinide orbitals. The method is therefore selectively sensitive to how actinide orbitals engage in bonding at the molecular level.

This project aims to develop RIXS into a quantitative tool to advance understanding of how actinides engage in chemical bonding. The extent and nature of bond-covalency is of particular importance since this profoundly influences physical properties, reactivity and the selectivity of actinide bond formation. There is much debate concerning the covalency of actinide bonding. On one hand, covalency can be considered as the mixing of electron density between atoms, and on the other covalency can be understood as occurring when the energy of the actinide and bonding atom orbitals match up. There are few experimental techniques with sensitivity to actinide covalency, and those that exist are limited to specific cases, i.e. only certain oxidation states, or types of bonded atom. RIXS has the potential to bridge the gap between the synthetic isolation of actinide compounds and first principle theory. Preliminary research by the PI has confirmed the sensitivity of RIXS to uranium bond covalency, identifying opportunities to advance understanding of actinide bonding.

The complex chemical bonding properties of actinides represent a major challenge to the nuclear energy sector. This project will develop methodologies and new knowledge that could lead to improved processes for the separation of actinides from other elements in nuclear waste processing and to better understand how actinides interact within the environment.

The proposed research requires an equal combination of RIXS measurement and theoretical simulations. Multiple levels of theory will be applied to identify the most accurate means to simulate spectra. Advances in synthetic chemistry have provided systematic families of compounds, that will be used to identify spectral trends, aiding the development of RIXS analysis. The focus will be on uranium and thorium, but the methods developed will be equally relevant to the study of transuranic compounds. The RIXS measurement methodologies, analysis and simulation methods developed will be distributed in an easy to use software package, to put the full potential of RIXS into the hands of the X-ray community.

Initial studies will explore donor covalency in single and double bonds to elucidate how RIXS spectral shape correlates with electronic structure. Less explored situations will be investigated, including compounds predicted as being highly covalent.

The developed RIXS analysis methods will then be applied to novel molecules prepared by collaborators and project partners. This will include a series of molecules that can adopt a variety of metal ions down a full column of the periodic table, such that our newfound understanding of U and Th bonding can be placed within the larger context of the periodic table. Finally, the complementary use of L and M-edge RIXS will be applied to pin down one of the most controversial and elusive problems in actinide electronic structure: the varying extent of 5f versus 6d orbital contributions to covalency.