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International audience; Although uranium–cerium dioxides are frequently used as a surrogate material for (U,Pu)O2−δ nuclear fuels, there is currently no reliable data regarding the oxygen stoichiometry and redox speciation of the cations in such samples. In order to fill this gap, this manuscript details a synchrotron study of highly homogeneous (U,Ce)O2±δ sintered samples prepared by a wet-chemistry route. HERFD-XANES spectroscopy led to determining accurately the O/M ratios (with M = U + Ce). Under a reducing atmosphere (pO2 ≈ 6 × 10–29 atm at 650 °C), the oxides were found to be close to O/M = 2.00, while the O/M ratio varied with the sintering conditions under argon (pO2 ≈ 3 × 10–6 atm at 650 °C). They globally appeared to be hyperstoichiometric (i.e., O/M > 2.00) with the departure from the dioxide stoichiometry decreasing with both the cerium content in the sample and the sintering temperature. Nevertheless, such a deviation from the ideal O/M = 2.00 ratio was found to generate only moderate structural disorder from EXAFS data at the U-L3 edge as all the samples retained the fluorite-type structure of the UO2 and CeO2 parent compounds. The determination of accurate lattice parameters owing to S-PXRD measurements led to complementing the data reported in the literature by various authors. These data were consistent with an empirical relation linking the unit cell parameter, the chemical composition, and the O/M stoichiometry, showing that the latter can be evaluated simply within a ± 0.02 uncertainty

Within the development of future nuclear reactors, wet chemistry routes have been investigated for the fabrication of advanced oxide fuels. In this frame, a multiparametric study focused on the hydrothermal conversion of uranium(IV) oxalate U(C2O4)2·nH2O into uranium oxides was undertaken in order to unravel the effects of temperature, pH, and kinetics. For pH ≤ 1, the lowest temperatures explored (typically from 180 to 200 °C) led to stabilized UO2+x/U4O9 mixtures exhibiting a global O/U ratio evaluated as 2.38 ± 0.10 from U M4-edge HERFD-XANES experiments. Higher temperatures (220-250 °C) led the oxide stoichiometry to decrease down to 2.13 ± 0.04 which corresponds to a lower fraction of U4O9 in the mixture. Additionally, increasing the temperature of the hydrothermal treatment efficiently improved the elimination of residual carbon species and water. Hydrothermal conversion of U(C2O4)2·nH2O also led to a drastic modification of the powders morphology. With this aim, pH tuning could be used to shift from bipyramidal aggregates (up to pH 1) to microspheres (2 ≤ pH ≤ 5) and then to nanometric powders (pH > 5). Finally, a kinetics study showed that uranium oxides can be obtained from the hydrothermal decomposition of oxalate within only few hours. If the samples collected early during the treatment always presented the characteristic XRD lines of UO2+x/U4O9 fluorite-type structure, then they were found to be strongly oxidized (O/U = 2.65 ± 0.14) which suggested the existence of a U(VI)-bearing amorphous secondary phase. The latter further tended to reduce through time. Hydrothermal conversion then probably proceeds as a two-step mechanism composed by the oxidative decomposition of uranium(IV) oxalate followed by the reduction of uranium by organic moieties and its hydrolysis. It appears as an easy and efficient way to yield highly pure uranium oxide samples in solution.

The effect of annealing on structural and thermochemical properties of a thorite-xenotime solid solution Th

Orthosilicates adopt the zircon structure types (

Hydrothermal conversion of thorium oxalate, Th(C

Zircon (ZrSiO

Standard solid-state methods produced black crystals of the compounds BaCu

A new ternary telluride, Ba

Single crystals and a polycrystalline sample of Ba

The black-colored compound BaUSe3 has been synthesized at 1173 K by a stoichiometric reaction of the elements in a CsCl flux. BaUSe3 crystallizes in the GdFeO3 structure type. There is no change in structure between 100 and 298 K. The U atoms in this structure are octahedrally connected to six Se atoms. Each octahedral unit shares all six corners with neighboring octahedra, forming a three-dimensional network. BaUSe3 can be charge balanced as Ba(2+)U(4+)(Se(2-))3. DFT electronic structure calculations found BaUSe3 to be antiferromagnetic in its ground state and to be a semiconductor with a band gap of 2.5 eV. The band gap is inconsistent with the black color of the material and with the small activation energy of 0.12(1) eV obtained from resistivity measurements. A UV-vis spectrum indicated that there was no band gap above 1 eV. It is possible that, for BaUSe3, intrinsic and extrinsic impurities from the flux create midgap states that lead to the experimentally measured narrow optical gap. More likely, BaUSe3 presents a challenge to DFT calculations as applied to 5f materials.

Multiparametric study of the hydrothermal synthesis of thorite, ThSiO

A multiparametric study of the hydrothermal synthesis of ThSiO

Single crystals of K(Th

International audience; Three new actinide thiophosphates, SrU(PS4)(2), BaU(PS4)(2), and SrTh(PS4)(2), have been synthesized by high-temperature solid-state methods, and their crystal structures Were determined from single-crystal. X-ray diffraction studies. These three isostructural compounds crystallize in a new structure type in space group D-4h(13)-P4(2)/mbc of the tetragonal system. Their structure features infinite one-dimensional chains of (1)(infinity)[An(PS4)(2)(2-)] anions (An = U or Th). Each An atom is coordinated by eight S atoms in a bicapped trigonal prism, and each P atom is tetrahedrally bonded to four S atoms. The compounds are readily charge :balanced as Ak(2+)An(4+)(P5+(S2-)(4))(2). Optical studies on single crystals of SrU(PS4)(2) and BaU(PS4)(2) as well as ground single crystals of SrTh(PS4)(2) revealed a direct band gap of 2.13(2) eV And an indirect band gap value of 1.99(2) eV for SrU(PS4)(2) and a,direct and indirect gap of about 2.28(2) eV for BaU(PS4)(2). SrTh(PS4)(2) has a relatively large band gap of 3.02(2) eV. DFT calculations foe SrU(PS4)(2) and BaU(PS4)(2) using the Ha functional predict both compounds to be antiferromagnetic and have very similar,electronic structures with band gaps of 2.7 eV. The band gap calculated for SrTh(PS4)(2) is 3.2 eV.

International audience; 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 D-3d(6) R3 (\₎c 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)(infinity)[MUQ(6) (6-)] 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 mu-Raman spectroscopic measurements and DFT calculations provide additional insight into these interesting subtle structural variations.

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 C2h5–P21/c of the monoclinic system, whereas BaU2Te5 adopts the (NH4)Pb2Br5 structure type in space group D4h18–I4/mcm of the tetragonal system. There are no Q–Q bonds in these structures, so the formulas charge balance as M2+(An4+)2(Q2–)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 behavio...

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.

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.

Dehydration of the hybrid compound [Ni3(OH)2(tp)2(H2O)4] (1) upon heating led to the sequential removal of coordinated water molecules to give [Ni3(OH)2(tp)2(H2O)2] (2) at T1 = 433 K and thereafter anhydrous [Ni2(OH)2(tp)] (3) at T2 = 483 K. These two successive structural transformations were thoroughly characterized by powder X-ray diffraction assisted by density functional theory calculations. The crystal structures of the two new compounds 2 and 3 were determined. It was shown that at T1 (433 K) the infinite nickel oxide chains built of the repeating structural unit [Ni3(μ3-OH)2](4+) in 1 collapse and lead to infinite porous layers, forming compound 2. The second transformation at T2 (483 K) gave the expected anhydrous compound 3, which is isostructural with Co2(OH)2(tp). These irreversible transitions directly affect the magnetic behavior of each phase. Hence, 1 was found to be antiferromagnetic at TN = 4.11 K, with metamagnetic behavior with a threshold field Hc of ca. 0.6 T. Compound 2 exhibits canted antiferromagnetism below TN = 3.19 K, and 3 is ferromagnetic below TC = 4.5 K.

Pure powdered compounds with a general formula Th

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.

International audience; Four new actinide chalcogenides—namely, Ba2Cu4USe6, Ba2Cu2ThSe5, Ba2Cu2USe5, and Sr2Cu2US5—were synthesized via solid-state methods at 1173 K. Single-crystal X-ray diffraction studies show that Ba2Cu4USe6 crystallizes in a new structure type in space group C2h5–P21/c of the monoclinic system, whereas the three other compounds are isostructural and adopt the Ba2Cu2US5 structure type in space group C2h3–C2/m, also of the monoclinic system. These Ak/Cu/An/Q structures (Ak = alkaline-earth metal; An = actinide; Q = chalcogen) have no short Q–Q interactions and, hence, are charge-balanced with Ak2+, Cu1+, An4+, and Q2–. Crystal structures of all these compounds are two-dimensional and feature layers that are separated by Ba2+ cations. The compositions of these layers differ. In the structure of Ba2Cu4USe6, the ∞2[Cu4USe64–] layers comprising USe6 octahedra and CuSe4 tetrahedra stack perpendicular to the a-axis. These ∞2[Cu4USe64–] layers show short Cu–Cu interactions. In the three isostructural Ak2Cu2AnQ5 compounds, AnQ6 octahedra and CuQ4 tetrahedra are connected along the c-axis in the sequence “...oct tet tet oct tet tet...” to form the ∞2[Cu2AnQ54–] layers. Resistivity, optical, and DFT calculations show semiconducting behavior for these compounds.

International audience; 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 D(2h)(17)Cmcm of the orthorhombic system with four formula units in a cell of dimensions a = 3.7500(8) angstrom, b = 12.110(2) angstrom, and c = 9.180(2) angstrom. 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 Sc3+U3+(S2-)(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.

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.

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 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.