Pentavalent and tetravalent uranium selenides, Tl₃Cu ₄USe₆ and Tl₂Ag₂USe₄: Syntheses, characterization, and structural comparison to other layered actinide chalcogenide compounds

The compounds Tl₃Cu₄USe₆ and Tl ₂Ag₂USe₄ were synthesized by the reaction of the elements in excess TlCl at 1123 K. Both compounds crystallize in new structure types, in space groups P2₁/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 an effective magnetic moment of 1.58(1) μ_B in the temperature range 72-300 K, whereas Tl ₂Ag₂USe₄ exhibits modified Curie-Weiss paramagnetic behavior with μ_eff = 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 ₃Cu₄USe₆ has Se bonding characteristic of discrete Se^2- 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 Tl₃Cu ₄USe₆ and +4 in Tl₂Ag₂USe ₄