Oxidation State of Uranium in A₆Cu₁₂U₂S₁₅ (A = K, Rb, Cs) Compounds

Black single crystals of A₆Cu₁₂U₂S₁₅ (A = K, Rb, Cs) have been synthesized by the reactive flux method. These isostructural compounds crystallize in the cubic space group Ia3̅d at room temperature. The structure comprises a three-dimensional framework built from US₆ octahedra and CuS₃ trigonal planar units with A cations residing in the cavities. There are no S–S bonds in the structure. To elucidate the oxidation state of U in these compounds, various physical property measurements and characterization methods were carried out. Temperature-dependent electrical resistivity measurement on a single crystal of K₆Cu₁₂U₂S₁₅ showed it to be a semiconductor. These three A₆Cu₁₂U₂S₁₅ (A = K, Rb, Cs) compounds all exhibit small effective magnetic moments, < 0.58 μ_B/U and band gaps of about 0.55(2) eV in their optical absorption spectra. From X-ray absorption near edge spectroscopy (XANES), the absorption edge of A₆Cu₁₂U₂S₁₅ is very close to that of UO₃. Electronic band structure calculations at the density functional theory (DFT) level indicate a strong degree of covalency between U and S atoms, but theory was not conclusive about the formal oxidation state of U. All experimental data suggest that the A₆Cu₁₂U₂S₁₅ family is best described as an intermediate U⁵⁺/U⁶⁺ sulfide system of (A⁺)₆(Cu⁺)₁₂(U⁵⁺)₂(S^2–)₁₃(S^–)₂ and (A⁺)₆(Cu⁺)₁₂(U⁶⁺)₂(S^2–)₁₅.