Rara Avis—U⁺⁵ in Chalcogenides

Chalcogen environments tend to stabilize low oxidation states, thus making the +5 oxidation state in uranium chalcogenides extremely rare: more than 97% of U-containing chalcogenides have oxidation states of U that do not exceed +4. Having one unpaired electron, the magnetism of U⁺⁵, 5f¹, is of significant interest; however, it is underexplored due to a scarcity of examples. This talk arranges around the novel structures of uranium sulfides containing U⁺⁵: Na₂Cu₅US₆, Na₃Cu₄US₆, and Na₃Cu₄USe₆; the impact of the uranium oxidation state on the properties will be highlighted. Firstly, I will discuss synthetic challenges to obtain those materials as well as provide support for the uranium oxidation states via bond valence sum analysis and X-ray absorption near edge spectroscopy. Second part of the talk will be focused on the magnetic properties of U⁺⁵-sulfides. Magnetic studies on powder and single crystal samples revealed that Na₂Cu₅US₆ is an antiferromagnet (T_N = 4.7 K) with anisotropic magnetic behavior. Moving from the Na₂Cu₅US₆ framework structure to the related layered Na₃Cu₄UQ₆ compositions does not change the magnetic behavior, and bulk measurements on the Na₃Cu₄UQ₆ powder showed that it is an antiferromagnet with T_N of 3.4 K (Q = S) and 5.9 K (Q = Se). The magnetic moments derived from the Curie-Weiss law for both Na₂Cu₅US₆ and Na₃Cu₄USe₆ samples were significantly reduced (1.06–1.08 ��_B) and cannot be described by either the spin-only or the total angular momentum models, highlighting the complexity of the interaction between 5f electrons and the ligands. Such a decrease in the magnetic moment can be explained by the covalency contribution of the U–Q bonding, leading to the changes in the electron density of uranium.