title: Oxidation state of uranium in A6Cu12U2S15 (A = K, Rb, Cs) compounds.
abstract: Black single crystals of A(6)Cu(12)U(2)S(15) (A = K, Rb, Cs) have been synthesized by the reactive flux method. These isostructural compounds crystallize in the cubic space group Ia ̅3d 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(6)Cu(12)U(2)S(15) showed it to be a semiconductor. These three A(6)Cu(12)U(2)S(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(6)Cu(12)U(2)S(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(6)Cu(12)U(2)S(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).
authors.affiliation: Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113, USA
authors.forename: Christos D
authors.initials: CD
authors.lastname: Malliakas
authors.forename: Jiyong
authors.initials: J
authors.lastname: Yao
authors.forename: Daniel M
authors.initials: DM
authors.lastname: Wells
authors.forename: Geng Bang
authors.initials: GB
authors.lastname: Jin
authors.forename: S
authors.initials: S
authors.lastname: Skanthakumar
authors.forename: Eun Sang
authors.initials: ES
authors.lastname: Choi
authors.forename: Mahalingam
authors.initials: M
authors.lastname: Balasubramanian
authors.forename: L
authors.initials: L
authors.lastname: Soderholm
authors.forename: Donald E
authors.initials: DE
authors.lastname: Ellis
authors.forename: Mercouri G
authors.initials: MG
authors.lastname: Kanatzidis
authors.forename: James A
authors.initials: JA
authors.lastname: Ibers