Black single crystals of A 6Cu 12U 2S 15 (A = K, Rb, Cs) have been synthesized by the reactive flux method. These isostructural compounds crystallize in the cubic space group Ia3̄d at room temperature. The structure comprises a three-dimensional framework built from US 6 octahedra and CuS 3 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 K 6Cu 12U 2S 15 showed it to be a semiconductor. These three A 6Cu 12U 2S 15 (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 A 6Cu 12U 2S 15 is very close to that of UO 3. 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 A 6Cu 12U 2S 15 family is best described as an intermediate U 5+/U 6+ sulfide system of (A +) 6(Cu +) 12(U 5+) 2(S 2-) 13(S -) 2 and (A +) 6(Cu +) 12(U 6+) 2(S 2-) 15.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry