Valence Fluctuations and Metallic Behavior in K6Cu12U2S15, a New Quaternary Sulfide with a Unique Three-Dimensional Cubic Framework

This is the authors' abstract. We don't add key points for this paper.

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 Ia3¯d (no. 230) with a=18.642(7) Å. 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 μeff of 2.96 μB per formula. This corresponds to 2.1 μB per U atom, and it is insufficient to distinguish between U4+ and U5+ paramagnetic ions. The conductivity of hot-pressed polycrystalline pellets is ∼1500 S cm−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 μV K−1 from 100–300 K) also confirm p-type metallic behavior.