Crystal structure and magnetic properties of the diuranium tritelluride U2Te3

Pentavalent and tetravalent uranium selenides, Tl3Cu4USe6 and Tl2Ag2USe4: syntheses, characterization, and structural comparison to other layered actinide chalcogenide compounds

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(+) 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 Curie-Weiss paramagnetic behavior with an effective magnetic moment of 1.58(1) μ(B) in the temperature range 72-300 K, whereas Tl(2)Ag(2)USe(4) 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(3)Cu(4)USe(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(3)Cu(4)USe(6) and +4 in Tl(2)Ag(2)USe(4).

Cu(x)UTe3: stabilization of UTe3 in the ZrSe3 structure type via copper insertion. The artifact of Te-Te chains and evidence for distortions due to long range modulations

The ternary phase, Cu(x)UTe3 (x = 0.25 and 0.33), was synthesized from a 3/1/4 mixture of Cu/U/Te that was heated to 800 degrees C for 6 days and cooled at a rate of -4 degrees C x h(-1). It adopts the monoclinic space group P2(1)/m with a = 6.0838(12) A, b = 4.2140(8) A, c = 10.361(2) A, beta = 98.83(3) degrees, and V = 262.47(9) A3 (for x approximately 0.25). The structure is built from UTe3 layers of ZrSe3-type that are connected in the [001] direction by Cu atoms. The Cu atoms stabilize alpha-UTe3 by inserting between the layers. Cu(x)UTe3 can be prepared rationally via a soft chemistry route by reaction of Cu with alpha-UTe3. The structural analysis suggests the presence of straight chains of Te atoms ( approximately 3.0 A apart) along the a-axis but this is an artifact as shown by electron diffraction studies of Cu(x)UTe3 that indicate the existence of a supercell along the a-axis. Pair distribution function analysis (PDF) was used to show that the Te-Te chains contain Te-Te dimers at 2.74 A. Charge transport measurements suggest a narrow gap semiconductor but they also indicate anomalous behavior as a function of temperature with a n-type to p-type transition at approximately 40 K.