Solid-state chemistry of uranium halides and chalcogenides: Syntheses, structures, and physical properties

Two novel quaternary uranium halides, [Hg3Te2][UCl 6] and [Hg4As2][UCl6], have been synthesized and their structures determined by single-crystal X-ray diffraction analysis. [Hg3Te2][UCl6] has a two-dimensional structure with 2infinity Hg3Te22+ layers, whereas [Hg4As2][UCl6] has a three-dimensional framework of 2infinity Hg3As2 layers interconnected by Hg2+ linearly bonded to As atoms. Both structures contain discrete [UCl6]2- anions between the layers. [Hg3Te2][UCl6] exhibits temperature-independent paramagnetism. The optical absorption spectra of these compounds reveal f-f transitions.;The new layered uranium chalcogenides AAuUQ3 (A = Rb, Cs; Q = Se, Te) and Cs2Hg2USe5 were synthesized via the reactive flux method. Both RbAuUSe3 and Cs2Hg 2USe5 display Curie-Weiss paramagnetic behavior, with effective magnetic moments mueff equal to 3.81(9) muB and 3.71(2) muB, respectively. The room-temperature electrical conductivity of Cs2Hg2USe5 is 3 x 10-3 S cm-1 in the [010] direction. The formal oxidation state of U is assigned as +4 in these compounds.;The compounds AU2Se6 (A = Rb, Tl) and Cs0.88 La0.68U1.32Se6, belonging to the KTh 2Se6 structure type, have been prepared by a solid-state route. Several structural features of these compounds have been investigated, including the Q--Q interactions and the disorder of the alkali metal cations. RbU2Se6 obeys the Curie-Weiss law above 30 K, with an effective magnetic moment mueff of 2.651(8) mu B/U. This compound also exhibits weak semiconducting behavior in the [100] direction; the conductivity at 298 K is 6 x 10-5 S cm-1.;Ba8Hg3U3S18, which crystallizes in a new structure type, was synthesized using the salt flux method. This material contains infinite chains of US6 octahedra and [S--Hg--S] 2- dithiomercurate anions, separated by Ba2+ cations. Ba8Hg3U3S18 displays an antiferromagnetic transition at TN = 59 K. On the basis of bond distance arguments and the magnetic susceptibility data, an oxidation state for U intermediate between +4 and +5 is proposed.;The compound La2U2Se9 was obtained in high yield from the stoichiometric reaction of the elements in an Sb 2Se3 flux. This is the first example of an ordered lanthanide/actinide with a novel structure type. La2U2Se9 displays an antiferromagnetic transition at TN = 5 K, but the low temperature specific heat of La2U2Se9 exhibits no anomalous behavior near the Neel temperature. Arguments are presented in favor of formal oxidation states of +3 for La and +4 for uranium.;Tl3Cu4USe6 and Tl2Ag 2USe4 were synthesized via a solid-state reaction of the elements with excess TlCl. In these respective compounds, 2infinity&sqbl0;Cu 4USe63-&sqbr0; and 2infinity&sqbl0;Ag 2USe42-&sqbr0; layers are separated by Tl+ cations. The magnetic susceptibility data of Tl3Cu4USe6 reveals magnetic transitions at both 5 and 70 K, whereas Tl2Ag2USe4 displays paramagnetic behavior. The formal oxidation state of U in Tl2Ag 2USe4 is +4, whereas it has been determined that Tl 3Cu4USe6 is a rare example of U5+.