Ba8Hg3U3S18: A Complex Uranium(+4)/Uranium(+5) Sulfide

The compound Ba8Hg3U3S18 was obtained from the solid-state reaction at 1123 K of U, HgS, BaS, and S, with BaBr2/KBr or BaCl2 as a flux. This material crystallizes in a new structure type in space group P (6) over bar of the hexagonal system with three formula units in a cell of dimensions a = 27.08(1) angstrom, c = 4.208(2) A, and V = 2673(2) angstrom(3). The structure contains infinite chains of US6 octahedra and nearly linear [S-Hg-S](2-) dithiomercurate anions, separated by Ba2+ cations. In the temperature range 100-300 K, the paramagnetic behavior of Ba8Hg3U3S18 can be fit to the Curie-Weiss law, resulting in mu(eff) = 5.40(4) mu(B), or 3.12(2) mu(B)/U. The compound displays an antiferromagnetic transition at T-N = 59 K. Although the formal oxidation states of Ba, Hg, and S can be assigned as +2, +2, and -2, the oxidation state of U is less certain. On the basis of interatomic distance arguments and the magnetic susceptibility data, the compound is proposed to contain U in both +4 and +5 formal oxidation states.