Adel Mesbah

Ba2An(S2)2S2 (An = U, Th): Syntheses, structures, optical, and electronic properties

The compounds Ba2An(S2)2S2 (An = U, Th) have been synthesized by reactions of the elements with BaS and S at 1273 and 1173 K, respectively. These isostructural compounds crystallize in a new structure type in the tetragonal space group D(4h)(15)-P42/nmc. The structure comprises Ba2+ cations and (∞)(2)[An(S2)2(S)2(4–)] layers. The An4+ cations in these layers are arranged linearly and are bridged by S2– anions. Coordination about the An center, which has symmetry 4m2, consists of two S2(2–) ions and four S2– ions. Thus, the compounds are charge-balanced with An4+. No other alkali-metal actinide chalcogenides are known that contain chalcogen–chalcogen bonds. Optical measurements on Ba2Th(S2)2S2 indicate a direct band gap of 2.46(5) eV. Density functional theory calculations, performed with the HSE exchange-correlation potential, lead to band gaps of 2.2 and 1.8 eV for Ba2Th(S2)2S2 and Ba2U(S2)2S2, respectively, thus demonstrating the utility of applying this functional to 5f-electron systems.

Syntheses, Crystal Structures, Transport Properties, and Theoretical Studies of Five Members of the MAn2Q5 Family: SrU2S5, BaU2Se5, PbU2S5, BaTh2S5, and BaU2Te5

Five compounds of the MAn2Q5 family, namely, SrU2S5, BaU2Se5, PbU2S5, BaTh2S5, and BaU2Te5, have been synthesized by high-temperature solid-state reactions. The crystal structures of these compounds were determined by single-crystal X-ray diffraction studies. SrU2S5, BaU2Se5, PbU2S5, and BaTh2S5 crystallize in the PbU2Se5 structure type in space group C2h(5)–P2(1)/c of the monoclinic system, whereas BaU2Te5 adopts the (NH4)Pb2Br5 structure type in space group D4h(18)–I4/mcm of the tetragonal system. There are no Q–Q bonds in these structures, so the formulas charge balance as M(2+)(An(4+))2(Q(2–))5. The An atoms in the monoclinic structure are seven- or eight-coordinated by Q atoms; the U atoms in the tetragonal structure are eight-coordinated. The M atoms in the monoclinic structure are coordinated to either eight or nine Q atoms, depending on the monoclinic β angle; the M atoms in the tetragonal structure are 10-coordinated. Resistivity studies on single crystals of SrU2S5, BaU2Se5, and PbU2S5 show metallic behavior with resistivities of 0.24, 10, and 3.3 mΩ·cm, respectively, at 298 K. Spin-polarized density functional theory in the generalized gradient approximation applied to the four U compounds suggests that they are ferromagnetic. In each compound, the density of states of one spin channel is found to be finite at the Fermi level, whereas there is a gap in the density of states of the other spin channel; this is characteristic of a half-metal.

Syntheses, structures, and electronic properties of Ba3FeUS6 and Ba3AgUS6.

The compounds Ba3FeUS6 and Ba3AgUS6 have been synthesized by the reactions of BaS, U, S, and M (= Fe or Ag) at 1223 K. These two isostructural compounds crystallize in the K4CdCl6 structure type in the trigonal system in space group D3d(6)–R3c. Both structures feature infinite ∞(1)[MUS6(6–)] chains along c that are separated by Ba atoms. The ∞(1)[FeUS6(6–)] chains are formed by the face-sharing of US6 trigonal prisms with FeS6 octahedra; in contrast, the ∞(1)[AgUS6(6–)] chains are formed by the face-sharing of US6 octahedra with AgS6 trigonal prisms. The Ba3FeUS6 compound charge balances with 3 Ba(2+), 1 Fe(2+), 1 U4+, and 6 S(2–), whereas Ba3AgUS6 charge balances with 3 Ba(2+), 1 Ag(1+), 1 U(5+), and 6 S(2–). This structure offers a remarkable flexibility in terms of the oxidation state of the incorporated uranium depending on the oxidation state of the d-block metal. DFT calculations performed with the HSE functional have led to band gaps of 2.3 and 2.2 eV for Ba3FeUS6 and Ba3AgUS6, respectively. From resistivity measurements, the Arrhenius activation energies are 0.12(1) and 0.43(1) eV for Ba3FeUS6 and Ba3AgUS6, 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.

The Flexible Ba7UM2S12.5O0.5 (M = V, Fe) Compounds: Syntheses, Structures and Spectroscopic, Resistivity, and Electronic Properties

Two new compounds, Ba7UV2S12.5O0.5 and Ba7UFe2S12.5O0.5, have been synthesized in fused-silica tubes by the direct combinations of V or Fe with U, BaS, and S at 1223 K. The compound Ba7UV2S12.5O0.5 crystallizes at 100 K in the Cs7Cd3Br17 structure type in space group D4h(18)–I4/mcm of the tetragonal system. The compound Ba7UFe2S12.5O0.5 crystallizes at 100 K in space group D4h(5)–P4/mbm of the tetragonal system. The structures are very similar with V/S or Fe/S networks in which Ba atoms reside as well as channels large enough to accommodate additional Ba atoms and infinite linear US5O chains. Each U atom is octahedrally coordinated to four equatorial S atoms, one axial S atom, and one axial O atom. The Fe/S network contains a S–S single bond, whereas the V/S network does not. The result is that the Fe3+ compound charge balances with 7 Ba2+, U4+, 2 Fe3+, 10.5 S2–, S2(2–), and 0.5 O2–, whereas the V4+ compound charge balances with 7 Ba2+, U4+, 2 V4+, 12.5 S2–, and 0.5 O2–. Other differences between these two compounds have been characterized by Raman spectroscopy and resistivity measurements. DFT calculations have provided insight into the nature of their bonding. The overall structural motif of Ba7UV2S12.5O0.5 and Ba7UFe2S12.5O0.5 offers a remarkable flexibility in terms of the oxidation state of the incorporated transition metal.

Three New Quaternary Actinide Chalcogenides Ba2TiUTe7, Ba2CrUTe7, and Ba2CrThTe7: Syntheses, Crystal Structures, Transport Properties, and Theoretical Studies

The three new quaternary actinide chalcogenides Ba2TiUTe7, Ba2CrUTe7, and Ba2CrThTe7 have been synthesized. From single-crystal X-ray diffraction studies these isostructural compounds are found to crystallize in a new structure type in space group D2h16–Pnma of the orthorhombic system. The structure features ∞1[MAnTe74–] strips (M = Cr or Ti; An = Th or U) that propagate in the b-direction and are separated by Ba cations. An atoms are coordinated to eight Te atoms in a bicapped trigonal-prismatic geometry while M atoms are octahedrally coordinated to six Te atoms. Sharing of the AnTe8 and MTe6 polyhedra forms ∞1[MAnTe74–] strips. The presence of the infinite linear Te–Te–Te chains in these compounds makes assignment of oxidation states arbitrary. Resistivity measurements and DFT calculations provide further insight into the properties of these compounds.

Positional flexibility: Syntheses and characterization of six uranium chalcogenides related to the 2H hexagonal perovskite family

Six new uranium chalcogenides, Ba4USe6, Ba3FeUSe6, Ba3MnUSe6, Ba3MnUS6, Ba3.3Rb0.7US6, and Ba3.2K0.8US6, related to the 2H hexagonal perovskite family have been synthesized by solid-state methods at 1173 K. These isostructural compounds crystallize in the K4CdCl6 structure type in space group D3d6–R3̅c of the trigonal system with six formula units per cell. This structure type is remarkably flexible. The structures of Ba3FeUSe6, Ba3MnUSe6, and Ba3MnUS6 consist of infinite ∞1[MUQ66–] chains (M = Fe or Mn; Q = S or Se) oriented along the c axis that are separated by Ba atoms. These chains are composed of alternating M-centered octahedra and U-centered trigonal prisms sharing triangular faces; in contrast, in the structures of Ba4USe6, Ba3.3Rb0.7US6, and Ba3.2K0.8US6, there are U-centered octahedra alternating with Ba-, Rb-, or K-centered trigonal prisms. Moreover, the Ba4USe6, Ba3FeUSe6, Ba3MnUSe6, and Ba3MnUS6 compounds contain U4+, whereas Ba3.3Rb0.7US6 and Ba3.2K0.8US6 are mixed U4+/5+ compounds. Resistivity and μ-Raman spectroscopic measurements and DFT calculations provide additional insight into these interesting subtle structural variations.

Caesium diuranium hexa­telluride

Single crystals of CsU2Te6 were synthesized from the reaction of U, Te, and Cs2Te3 at 1273u2005K. CsU2Te6 crystallizes in the space group Cmcm in the CsTh2Te6 structure type. The asymmetric unit comprises one U (site symmetry m2m), one Cs (m2m; half-occupancy) and two Te atoms (m.. and m2m). The structure of CsU2Te6 consists of infinite [U2Te6] layers perpendicular to [010] separated by Cs atoms. There are infinite Te—Te—Te linear chains along [001].

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.