Rare Bird: U⁺⁵ in Uranium Chalcogenides

Chalcogen environments tend to stabilize low oxidation states, thus making the +5 oxidation state in uranium chalcogenides extremely rare. Having one unpaired electron, the magnetism of U+5, 5f1, is of significant interest; however, it is underexplored due to a scarcity of examples. Three related all-U+5 chalcogenides, Na2Cu5US6, Na3Cu4US6, and Na3Cu4USe6, were obtained as single crystals via the flux-assisted boron chalcogen mixture method as part of a broader investigation of the Na–Cu–U–Q (Q = S and Se) systems. To test the formation of the Se-analog of Na2Cu5US6, we performed in situ high-temperature powder X-ray diffraction using sealed capillaries; this approach demonstrated the formation and presence of only Na3Cu4USe6 and not “Na2Cu5USe6” in the reaction mixture. In addition to assigning oxidation states using charge balance and the sample composition, oxidation states of copper, +1, and uranium, +5, in Na2Cu5US6 were also determined via X-ray absorption near edge spectroscopy. Magnetic properties were investigated on both bulk powder samples of Na2Cu5US6 and Na3Cu4UQ6 (Q = S and Se) and millimeter-size single crystals of Na2Cu5US6, where magnetic susceptibility and magnetization vs field measurements revealed antiferromagnetic behavior of Na2Cu5US6 and Na3Cu4UQ6 (Q = S and Se) with a TN of 4.7, 3.4, and 5.9 K, respectively. Finally, a broad discussion of the effective magnetic moment of U+5, which is significantly smaller, 1.06 μB in Na2Cu5US6 and 1.08 μB in Na3Cu4USe6, than predicted by spin-only (1.73 μB) and total angular momentum (2.54 μB) models. The fact that only about 20 compositions of such materials are currently known, highlights the need to explore additional compositions containing uranium in rare oxidation states before it will be possible to understand the magnetic properties of such materials more fully.