Abstract:
Thanks to its 5f orbitals, uranium benefits from several valence states in inorganic compounds, from U2+ to U6+, and possesses a wide crystal chemistry. The radial expansion of these orbitals leads to energetically close crystal field (dominating in the case of 3d elements) and spin-orbit coupling (dominating in the case of 4f rare earths) interactions, and result in rich and exotic physical properties1 (e.g. coexistence of superconductivity and ferromagnetism). When associated with a chalcogen element (Q = S, Se, Te), uranium forms inorganic compounds characterized by various crystallographic structures leading to unique uranium polyhedral environments,2 with e.g. the presence of (S2)2- dimers.
In this context, binary and ternary uranium sulphides were synthetized at high temperatures in sealed evacuated silica tubes. Their crystal chemistry was then investigated in order to better understand the relationship between uranium oxidation state, chemical bonding and physical properties. High-resolution synchrotron X-ray spectroscopies reveal a predominance of U4+ together with mixed U3+/U4+ configurations in specific crystallographic environments, and highlight the more covalent character of the U-S bond compared to U-O. The investigation was extended to several families of ternary uranium sulphides U-TM-S, where TM is a 3d transition metal,3 illustrating how crystal chemistry can stabilize different uranium valence states depending on the local structural environment. The crystal chemistry of selected examples of compounds will be discussed in parallel to their physical properties.
References
(1) Aoki, D.; Brison, J.-P.; Flouquet, J.; Ishida, K.; Knebel, G.; Tokunaga, Y.; Yanase, Y. Unconventional Superconductivity in UTe2. J. Phys.: Condens. Matter 2022, 34 (24), 243002. https://doi.org/10.1088/1361-648X/ac5863.
(2) Mesbah, A.; Prakash, J.; Ibers, J. A. Overview of the Crystal Chemistry of the Actinide Chalcogenides: Incorporation of the Alkaline-Earth Elements. Dalton Trans. 2016, 45 (41), 16067–16080. https://doi.org/10.1039/C6DT02540D.
(3) Noël, H.; Troć, R. Magnetic Properties of Mixed Uranium and 3d Element Chalcogenides of the MU8X17 Type. J. Solid State Chem. 1979, 27 (2), 123–135.
https://doi.org/10.1016/0022-4596(79)90151-8.