Pentavalent and Tetravalent Uranium Selenides, Tl3Cu4USe6 and Tl2Ag2USe4: Syntheses, Characterization, and Structural Comparison to Other Layered Actinide Chalcogenide Compounds

The compounds Tl3Cu4USe6 and Tl2Ag2USe4 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 USe6 octahedra and MSe4 (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. Tl3Cu4USe6 displays peaks in its magnetic susceptibility at 5 and 70 K. It exhibits modified Curie Weiss paramagnetic behavior with in effective magnetic moment of 1.58(1) mu(B) in the temperature range 72-300 K, whereas Tl2Ag2USe4 exhibits modified Curie-Weiss Paramagnetic behavior with mu(eff) = 3.4(1) mu(B) in the temperature 100-300 K. X-ray absorption near-edge structure (XANES) results from scanning transmission X-ray spectromicroscopy confirm that Tl2Cu4USe6 has Se bonding characteristic of discrete Se2- units, Cu bonding generally representative of Cu+, and U bonding consistent with a U4+ or U5+ species. On the basis of these measurements, as well as bonding arguments, the formal oxidation states for U may be assigned as +5 in Tl3Cu4USe6 and +4 in Tl2Ag2USe4.