Published: June 16, 2011 r2011 American Chemical Society 6656 dx.doi.org/10.1021/ic200565n | Inorg. Chem. 2011, 50, 6656–6666 ARTICLE pubs.acs.org/IC Pentavalent and Tetravalent Uranium Selenides, Tl 3 Cu 4 USe 6 and Tl 2 Ag 2 USe 4 : Syntheses, Characterization, and Structural Comparison to Other Layered Actinide Chalcogenide Compounds Daniel E. Bugaris, † Eun Sang Choi, ‡ Roy Copping, § Per-Anders Glans, || Stefan G. Minasian, §,^ Tolek Tyliszczak, || Stosh A. Kozimor, ^ David K. Shuh, § and James A. Ibers* ,† † Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3113, United States ‡ Department of Physics and National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32310, United States § Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ) Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ^ Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States b S Supporting Information ’ INTRODUCTION The vast majority of quaternary uranium chalcogenides (Q = chalcogen = S, Se, Te) also incorporate a transition metal and an alkali metal or an alkaline-earth metal. 1 Theoretical calculations for the AMUQ 3 (A = K, Rb, Cs; M = Cu, Ag; Q = S, Se) compounds show that the electrons from A make little contribu- tion to the density of states (DOS) near the Fermi level. 2 Furthermore, it is to be expected that an s-block element will have a negligible effect on the magnetic behavior of a compound. In the search for new quaternary uranium chalcogenides, it is desirable to replace the alkali metal or alkaline-earth metal with a p-block element, or ultimately, a second transition metal or a lanthanide. The most obvious candidate for achieving this goal is to replace the alkali metal with thallium. Tl prefers the þ1 oxidation state similar to the alkali metals and exhibits other chemical similarities as well. 3 However, there are two significant differ- ences between Tl and the alkali metals. First, because the electro- negativity of Tl is much larger than that of the alkali metals (2.04 vs 0.98 for Li), 4 the bonding in the resultant compound should be more covalent. Second, Tl þ possesses a lone pair of electrons that can display stereoactivity, a feature lacking for the alkali metals. Both of these differences may contribute to more complex structures in the quaternary uranium chalcogenide system. There is precedent for such ternary phases, such as TlU 2 Se 6 5 and Tl 0.56 UTe 3 . 6 We report here the syntheses, structures, and characterization of the new compounds Tl 3 Cu 4 USe 6 and Tl 2 Ag 2 USe 4 as well as provide a comparison to known layered quaternary actinide chalcogenides. We present evidence that the formal oxidation state of U is þ5 in Tl 3 Cu 4 USe 6 and þ4 in Tl 2 Ag 2 USe 4 . ’ EXPERIMENTAL METHODS Syntheses. Caution! Great care must be exercised when handling Tl and its compounds. When exposed to moisture, Tl forms TlOH, which is soluble in water and is easily absorbed through the skin. Thus, contact with Received: March 18, 2011 ABSTRACT: The compounds Tl 3 Cu 4 USe 6 and Tl 2 Ag 2 USe 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 þ 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 3 Cu 4 USe 6 displays peaks in its magnetic susceptibility at 5 and 70 K. It exhibits modified CurieÀWeiss paramagnetic behavior with an effective magnetic moment of 1.58(1) μ B in the temperature range 72À300 K, whereas Tl 2 Ag 2 USe 4 exhibits modified CurieÀWeiss para- magnetic behavior with μ eff = 3.4(1) μ B in the temperature range 100À300 K. X-ray absorption near-edge structure (XANES) results from scanning transmission X-ray spectro- microscopy confirm that Tl 3 Cu 4 USe 6 has Se bonding characteristic of discrete Se 2À units, Cu bonding generally representative of Cu þ , and U bonding consistent with a U 4þ or U 5þ species. On the basis of these measurements, as well as bonding arguments, the formal oxidation states for U may be assigned as þ5 in Tl 3 Cu 4 USe 6 and þ4 in Tl 2 Ag 2 USe 4