Transuranium Sulfide via the Boron Chalcogen Mixture Method and Reversible Water Uptake in the NaCuTS₃ Family

The behavior of 5f electrons in soft ligand environments makes actinides, and especially transuranium chalcogenides, an intriguing class of materials for fundamental studies. Due to the affinity of actinides for oxygen, however, it is a challenge to synthesize actinide chalcogenides using non-metallic reagents. Using the boron chalcogen mixture method, we achieved the synthesis of the transuranium sulfide NaCuNpS₃ starting from the oxide reagent, NpO₂. Via the same synthetic route, the isostructural composition of NaCuUS₃ was synthesized and the material contrasted with NaCuNpS₃. Single crystals of the U-analogue, NaCuUS₃, were found to undergo an unexpected reversible hydration process to form NaCuUS₃·xH₂O (x ≈ 1.5). A large combination of techniques was used to fully characterize the structure, hydration process, and electronic structures, specifically a combination of single crystal, powder, high temperature powder X-ray diffraction, extended X-ray absorption fine structure, infrared, and inductively coupled plasma spectroscopies, thermogravimetric analysis, and density functional theory calculations. The outcome of these analyses enabled us to determine the composition of NaCuUS₃·xH₂O and obtain a structural model that demonstrated the retention of the local structure within the [CuUS₃]^- layers throughout the hydration-dehydration process. Band structure, density of states, and Bader charge calculations for NaCuUS₃, NaCuUS₃·xH₂O, and NaCuNpS₃ along with X-ray absorption near edge structure, UV-vis-NIR, and work function measurements on ACuUS₃ (A = Na, K, and Rb) and NaCuUS₃·xH₂O samples were carried out to demonstrate that electronic properties arise from the [CuTS₃]^- layers and show surprisingly little dependence on the interlayer distance.