Matthew D. Ward

Syntheses and characterization of six quaternary uranium chalcogenides A2M4U6Q17 (A = Rb or Cs; M = Pd or Pt; Q = S or Se).

The A(2)M(4)U(6)Q(17) compounds Rb(2)Pd(4)U(6)S(17), Rb(2)Pd(4)U(6)Se(17), Rb(2)Pt(4)U(6)Se(17), Cs(2)Pd(4)U(6)S(17), Cs(2)Pd(4)U(6)Se(17), and Cs(2)Pt(4)U(6)Se(17) were synthesized by the high-temperature solid-state reactions of U, M, and Q in a flux of ACl or Rb(2)S(3). These isostructural compounds crystallize in a new structure type, with two formula units in the tetragonal space group P4/mnc. This structure consists of a network of square-planar MQ(4), monocapped trigonal-prismatic UQ(7), and square-antiprismatic UQ(8) polyhedra with A atoms in the voids. Rb(2)Pd(4)U(6)S(17) is a typical semiconductor, as deduced from electrical resistivity measurements. Magnetic susceptibility and specific heat measurements on single crystals of Rb(2)Pd(4)U(6)S(17) show a phase transition at 13 K, the result either of antiferromagnetic ordering or of a structural phase transition. Periodic spin-polarized band structure calculations were performed on Rb(2)Pd(4)U(6)S(17) with the use of the first principles DFT program VASP. Magnetic calculations included spin-orbit coupling. With U f-f correlations taken into account within the GGA+U formalism in calculating partial densities of states, the compound is predicted to be a narrow-band semiconductor with the smallest indirect and direct band gaps being 0.79 and 0.91 eV, respectively.

Synthesis and characterization of two quaternary uranium tellurides, RbTiU3Te9 and CsTiU3Te9

Black crystals of RbTiU₃Te₉ and CsTiU₃Te₉ have been synthesized at 1223 and 1173 K, respectively, by high-temperature solid-state routes. These compounds crystallize in a new structure type in space group C(2h)²-P2₁/m of the monoclinic system. The structure, which is similar to that of CsTiUTe₅, consists of UTe₂ layers connected into a three-dimensional framework by TiTe₆ octahedra. The expanded UTe₂ layers leave channels that are filled by Rb or Cs atoms. Single-crystal resistivity measurements on CsTiU₃Te₉ are consistent with semiconducting behavior; the calculated activation energy is 0.30(1) eV. X-ray photoelectron spectroscopic measurements on CsTiU₃Te₉ indicate that the compound contains U⁴⁺. From single-crystal magnetic measurements, CsTiU₃Te₉ is consistent with antiferromagnetic coupling between magnetic U atoms. The very low value of the effective magnetic moment of 0.56(2) μ(B) is believed to arise from a coexistence of magnetic and nonmagnetic U atoms.

Synthesis and characterization of eight compounds of the MU8Q17 family: ScU8S17, CoU8S17, NiU8S17, TiU8Se17, VU8Se17, CrU8Se17, CoU8Se17, and NiU8Se17.

The solid-state MU8Q17 compounds ScU8S17, CoU8S17, NiU8S17, TiU8Se17, VU8Se17, CrU8Se17, CoU8Se17, and NiU8Se17 were synthesized from the reactions of the elements at 1173 or 1123 K. These isostructural compounds crystallize in space group C2h3 - C2/m of the monoclinic system in the CrU8S17 structure type. X-ray absorption near-edge structure spectroscopic studies of ScU8S17 indicate that it contains Sc3+, and hence charge balance is achieved with a composition that includes U3+ as well as U4+. The other compounds charge balance with M2+ and U4+. Magnetic susceptibility measurements on ScU8S17 indicate antiferromagnetic couplings and a highly reduced effective magnetic moment. Ab Initio calculations find the compound to be metallic. Surprisingly, the Sc–S distances are actually longer than all the other M–S interactions, even though the ionic radii of Sc3+, low-spin Cr2+, and Ni2+ are similar.

Synthesis and Structure of the [(UO2)S4]6– Anion: A Cation-Stabilized Uranyl Sulfide

The new uranyl sulfide anion [(UO2)S4](6-) has been synthesized and characterized as a cation-stabilized anion in the compound Na2Ba2(UO2)S4. This compound was synthesized at 873 K from the solid-state reaction of uranium, Na2O2, BaS, and sulfur. The coordination about the U(6+) center in [(UO2)S4](6-) is square bipyramidal with the uranyl O atoms 180° apart and four equatorial S atoms. The Na(+) and Ba(2+) cations form interactions with the uranyl O atoms. Despite the inherent difficulties involved in the synthesis of complex uranium oxysulfides, it is demonstrated that under the right reaction conditions the UO2(2+) species can be produced in situ and result in totally new chalcogen derivatives.

Synthesis, crystal structure, resistivity, magnetic, and theoretical study of ScUS3.

Single crystals of ScUS3 were synthesized in high yield in a single step at 1173 K. ScUS3 crystallizes in the FeUS3 structure type in the space group D2h(17)–Cmcm of the orthorhombic system with four formula units in a cell of dimensions a = 3.7500(8) Å, b = 12.110(2) Å, and c = 9.180(2) Å. Its structure consists of edge- and corner-sharing ScS6 octahedra that form two-dimensional layers. U atoms between layers are connected to eight S atoms in a bicapped trigonal-prismatic fashion. ScUS3 can be easily charge-balanced as Sc(3+)U(3+)(S(2–))3 as there are no S–S single bonds present in the crystal structure. High temperature-dependent resistivity measurements on a single crystal of ScUS3 show semiconducting behavior with an activation energy of 0.09(1) eV. A magnetic study on powdered single crystals of ScUS3 reveals an antiferromagnetic transition at 198 K followed by a ferromagnetic transition at 75 K. The weak ferromagnetic behavior at low temperature may originate from canted antiferromagnetic spins. A density functional theory (DFT) calculation predicts ScUS3 to be ferromagnetic and either a very poor metal or a semiconductor with a very small gap.

The [U2(μ-S2)2Cl8]4– Anion: Synthesis and Characterization of the Uranium Double Salt Cs5[U2(μ-S2)2Cl8]I

Red plates of Cs5[U2(μ-S2)2Cl8]I were obtained in good yield from the reaction at 1173 K of U, GeI2 or SnI4, and S, with CsCl flux. The compound crystallizes in space group D2h25-Immm of the orthorhombic system in the Cs5[Nb2(μ-S2)2Cl8]Cl structure type. The centrosymmetric [U2(μ-S2)2Cl8]4– anion in the structure has mmm symmetry with the two U4+ atoms separated by 3.747(1) Å. Each U atom is coordinated to four Cl atoms and four S atoms from two S22– groups in a square-antiprismatic arrangement. The polarized absorbance spectra of Cs5[U2(μ-S2)2Cl8]I display prominent optical anisotropy. Magnetic measurements are consistent with the modified Curie–Weiss law at high temperatures. The low-temperature behavior may arise from antiferromagnetic coupling of the U4+ ions within the anion.

The Synthesis and Characterization of Four New Uranium(IV) Chlorophosphates: UCl4(POCl3), [U2Cl9][PCl4], UCl3(PO2Cl2), and U2Cl8(POCl3)

The new uranium(IV) chlorophosphate compounds UCl4(POCl3) and [U2Cl9][PCl4] have been synthesized by the solid-state reactions of U, P2O5, and PCl5 at 648 K; the compounds UCl3(PO2Cl2) and U2Cl8(POCl3) have been synthesized at 648 K with the same reactants plus added S. Their structures are, respectively, chainlike, a simple salt, three-dimensional, and sheetlike. From ab initio calculations, U2Cl8(POCl3) and UCl3(PO2Cl2) are found to be ferromagnetic, whereas UCl4(POCl3) is found to be antiferromagnetic. U2Cl8(POCl3) is a strong metal, whereas UCl3(PO2Cl2) is a weaker metal. In contrast, UCl4(POCl3) has a finite band gap, with a value of 0.35 eV.