Rare Bird:\nU<sup>+5</sup> in Uranium Chalcogenides

Chalcogen environments tend to stabilize low oxidation\nstates,\nthus making the +5 oxidation state in uranium chalcogenides extremely\nrare. Having one unpaired electron, the magnetism of U<sup>+5</sup>, 5f<sup>1</sup>, is of significant interest; however, it is underexplored\ndue to a scarcity of examples. Three related all-U<sup>+5</sup> chalcogenides,\nNa<sub>2</sub>Cu<sub>5</sub>US<sub>6</sub>, Na<sub>3</sub>Cu<sub>4</sub>US<sub>6</sub>, and Na<sub>3</sub>Cu<sub>4</sub>USe<sub>6</sub>,\nwere obtained as single crystals via the flux-assisted boron chalcogen\nmixture method as part of a broader investigation of the Na–Cu–U–<i>Q</i> (<i>Q</i> = S and Se) systems. To test the formation\nof the Se-analog of Na<sub>2</sub>Cu<sub>5</sub>US<sub>6</sub>, we\nperformed <i>in situ</i> high-temperature powder X-ray diffraction\nusing sealed capillaries; this approach demonstrated the formation\nand presence of only Na<sub>3</sub>Cu<sub>4</sub>USe<sub>6</sub> and\nnot “Na<sub>2</sub>Cu<sub>5</sub>USe<sub>6</sub>” in\nthe reaction mixture. In addition to assigning oxidation states using\ncharge balance and the sample composition, oxidation states of copper,\n+1, and uranium, +5, in Na<sub>2</sub>Cu<sub>5</sub>US<sub>6</sub> were also determined via X-ray absorption near edge spectroscopy.\nMagnetic properties were investigated on both bulk powder samples\nof Na<sub>2</sub>Cu<sub>5</sub>US<sub>6</sub> and Na<sub>3</sub>Cu<sub>4</sub>U<i>Q</i><sub>6</sub> (<i>Q</i> = S and\nSe) and millimeter-size single crystals of Na<sub>2</sub>Cu<sub>5</sub>US<sub>6</sub>, where magnetic susceptibility and magnetization vs\nfield measurements revealed antiferromagnetic behavior of Na<sub>2</sub>Cu<sub>5</sub>US<sub>6</sub> and Na<sub>3</sub>Cu<sub>4</sub>U<i>Q</i><sub>6</sub> (<i>Q</i> = S and Se) with a <i>T</i><sub>N</sub> of 4.7, 3.4, and 5.9 K, respectively. Finally,\na broad discussion of the effective magnetic moment of U<sup>+5</sup>, which is significantly smaller, 1.06 μ<sub>B</sub> in Na<sub>2</sub>Cu<sub>5</sub>US<sub>6</sub> and 1.08 μ<sub>B</sub> in\nNa<sub>3</sub>Cu<sub>4</sub>USe<sub>6</sub>, than predicted by spin-only\n(1.73 μ<sub>B</sub>) and total angular momentum (2.54 μ<sub>B</sub>) models. The fact that only about 20 compositions of such\nmaterials are currently known, highlights the need to explore additional\ncompositions containing uranium in rare oxidation states before it\nwill be possible to understand the magnetic properties of such materials\nmore fully.