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 compound K6Cu12U2S15. Synthesized from the reaction of Cu and U in a molten alkali metal/polysulfide flux, the compound crystallizes in the cubic space group Ia3d (no. 230) with a = 18.642(7) angstroms. Its complex structure is built from [US6] octahedra connected into one-dimensional columns with [CuS3] trigonal planar units, which also serve as inter-column connection points for the extended three-dimensional structure. Magnetic susceptibility measurements reveal Curie-Weiss paramagnetic behavior at temperatures above 100 K with a microeff of 2.96 microB per formula. This corresponds to 2.1 microB per U atom, and it is insufficient to distinguish between U4+ and U5+ paramagnetic ions. The conductivity of hot-pressed polycrystalline pellets is approximately 1500 Scm(-1) at room temperature, and increasing conductivity with decreasing temperature is observed, indicating metallic behavior for the material. Small and positive values for the material's thermopower (3-4 microVK(-1) from 100-300 K) also confirm p-type metallic behavior.