Valence fluctuations and metallic behavior in K6Cu12U2S15, a new quaternary sulfide with a unique three-dimensional cubic framework

Mixed S2-/S1- oxidation states have been discovered in the new quaternary c ompound K6Cu12U2S15. Synthesized from the reaction of Cu and U in a molten alkali metal/polysulfide flux, the compound crystallizes in the cubic space group <Ia(3)over bar d> (no. 230) with a=18.642(7) Angstrom. Its complex s tructure is built from [US6] octahedra connected into one-dimensional colum ns with [CuS3] trigonal planar units, which also serve as inter-column conn ection points for the extended three-dimensional structure. Magnetic suscep tibility measurements reveal Curie - Weiss paramagnetic behavior at tempera tures above 100 K with a mu(eff) of 2.96 mu(B) per formula. This correspond s to 2.1 mu(B) per U atom, and it is insufficient to distinguish between U4 + and U5+ paramagnetic ions. The conductivity of hot-pressed polycrystallin e pellets is similar to 1500 S cm(-1) at room temperature, and increasing c onductivity with decreasing temperature is observed, indicating metallic be havior for the material. Small and positive values for the material's therm opower (3-4 mu VK-1 from 100-300 K) also confirm p-type metallic behavior.