Pentavalent and tetravalent uranium selenides, Tl_3Cu _4USe_6 and Tl_2Ag_2USe_4: Syntheses, characterization, and structural comparison to other layered actinide chalcogenide compounds

摘要

The compounds Tl_3Cu_4USe_6 and Tl _2Ag_2USe_4 were synthesized by the reaction of the elements in excess TlCl at 1123 K. Both compounds crystallize in new structure types, in space groups P2_1/c and C2/m, respectively, of the monoclinic system. Each compound contains layers of USe_6 octahedra and MSe_4 (M = Cu, Ag) tetrahedra, separated by Tl~+ cations. The packing of the octahedra and the tetrahedra within the layers is compared to the packing arrangements found in other layered actinide chalcogenides. Tl_3Cu_4USe_6 displays peaks in its magnetic susceptibility at 5 and 70 K. It exhibits modified Curie-Weiss paramagnetic behavior with an effective magnetic moment of 1.58(1) μB in the temperature range 72-300 K, whereas Tl _2Ag_2USe_4 exhibits modified Curie-Weiss paramagnetic behavior with μeff = 3.4(1) μ_B in the temperature range 100-300 K. X-ray absorption near-edge structure (XANES) results from scanning transmission X-ray spectromicroscopy confirm that Tl _3Cu_4USe_6 has Se bonding characteristic of discrete Se~(2-) units, Cu bonding generally representative of Cu ~+, and U bonding consistent with a U~(4+) or U~(5+) species. On the basis of these measurements, as well as bonding arguments, the formal oxidation states for U may be assigned as +5 in Tl_3Cu _4USe_6 and +4 in Tl_2Ag_2USe _4