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.硫族元素（Chalcogen）配位环境通常可稳定低氧化态，因此铀的硫族化合物中+5氧化态极为罕见。五价铀（U⁵⁺）拥有1个未成对电子，其5f¹构型的磁学性质备受关注，但由于相关实例匮乏，该方向的研究尚不充分。本研究作为Na–Cu–U–Q（Q=S、Se）体系系统性探究的一部分，通过硼硫族混合助熔剂法成功合成了三种全含五价铀的硫族化合物单晶：Na₂Cu₅US₆、Na₃Cu₄US₆与Na₃Cu₄USe₆。为验证Na₂Cu₅US₆的硒类似物能否被合成，我们采用密封毛细管开展了原位高温粉末X射线衍射测试，结果显示反应体系中仅生成了Na₃Cu₄USe₆，并未得到预期的"Na₂Cu₅USe₆"。除通过电荷平衡与样品组成推断氧化态外，我们还借助X射线吸收近边谱（X-ray absorption near edge spectroscopy, XANES）确定了Na₂Cu₅US₆中铜（+1价）与铀（+5价）的氧化态。我们对Na₂Cu₅US₆、Na₃Cu₄UQ₆（Q=S、Se）的块体粉末样品，以及Na₂Cu₅US₆的毫米级单晶开展了磁学性质表征：通过磁化率与磁化强度-磁场曲线测试，发现这三类化合物均表现出反铁磁行为，其奈尔温度（Neel temperature, T_N）分别为4.7 K、3.4 K与5.9 K。最后，我们针对五价铀的有效磁矩展开了详细讨论：在Na₂Cu₅US₆与Na₃Cu₄USe₆中，其有效磁矩分别为1.06 μB与1.08 μB，远低于仅自旋模型（1.73 μB）与总角动量模型（2.54 μB）的预测值。目前已知的这类铀基硫族化合物仅约20种组分，这凸显出：若要更全面地理解这类材料的磁学性质，亟需探索更多包含罕见氧化态铀的化合物组分。