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

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posted on 04.06.2012, 00:00 authored by Christos D. Malliakas, Jiyong Yao, Daniel M. Wells, Geng Bang Jin, S. Skanthakumar, Eun Sang Choi, Mahalingam Balasubramanian, L. Soderholm, Donald E. Ellis, Mercouri G. Kanatzidis, James A. Ibers
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 Iad 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 μB/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.

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