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, 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(S2–)13(S–)2 and (A+)6(Cu+)12(U6+)2(S2–)15.