Search

This work is the first attempt to prepare Nd1−xCaxUxPO4 monazite–cheralite with 0 < x ≤ 0.1 by a wet chemistry method. This method relies on the precipitation under hydrothermal conditions (T = 110 °C for four days) of the Nd1−xCaxUxPO4·nH2O rhabdophane precursor, followed by its thermal conversion for 6 h at 1100 °C in air or Ar atmosphere. The optimized synthesis protocol led to the incorporation of U and Ca in the rhabdophane structure. After heating at 1100 °C for 6 h in air, single-phase monazite–cheralite samples were obtained. However, α-UP2O7 was identified as a secondary minor phase in the samples heated under Ar atmosphere. The U speciation in the samples converted in an oxidising atmosphere was carefully characterized using synchrotron radiation by combining HERFD-XANES and XRD. These results showed the presence of a minor secondary phase containing hexavalent uranium and phosphate with a stoichiometry of U : P = 0.78. This highly labile uranyl phosphate phase incorporated 21 mol% of the uranium initially precipitated with the rhabdophane precursor. This phase was completely removed by a washing protocol. Thus, single-phase monazite–cheralite was obtained through the wet chemistry route described in this work with a maximum U loading of x = 0.08. [ABSTRACT FROM AUTHOR]

Black single crystals of Cs2U2(P2Se9)(Se2)2 were synthesized at 1023 K by a solid‐state reaction. Its structure was determined by single‐crystal X‐ray diffraction methods. The compound is isostructural with Rb2U2(P2Se9)(Se2)2 and Cs2Th2(P2Se9)(Se2)2 compounds, and crystallizes in the orthorhombic space group Pbca. Cs2U2(P2Se9)(Se2)2 displays a three‐dimensional structure comprising USe9 polyhedra connected via the Se22– and P2Se9 species with channels filled by Cs atoms. From electronic structure calculations the compound is found to be a metal with a ferromagnetic arrangement of the magnetic moments. [ABSTRACT FROM AUTHOR]

Black single crystals of Cs2U2(P2Se9)(Se2)2 were synthesized at 1023 K by a solid‐state reaction. Its structure was determined by single‐crystal X‐ray diffraction methods. The compound is isostructural with Rb2U2(P2Se9)(Se2)2 and Cs2Th2(P2Se9)(Se2)2 compounds, and crystallizes in the orthorhombic space group Pbca. Cs2U2(P2Se9)(Se2)2 displays a three‐dimensional structure comprising USe9 polyhedra connected via the Se22– and P2Se9 species with channels filled by Cs atoms. From electronic structure calculations the compound is found to be a metal with a ferromagnetic arrangement of the magnetic moments. [ABSTRACT FROM AUTHOR]

Crystallographic study was performed on the zinc undecanoate Zn(C 11 H 21 O 2 ) 2 by X-ray diffraction. Three polymorphs A , B and C were identified. The structures of A and B were solved from X-ray diffraction on platelets like single crystals obtained by crystallization in methanol. Polymorph A: SG = P2 1 /c, a = 54.411(4) Å, b = 4.666(4) Å, c = 9.2231(8) Å, β = 92.54(7)°, V = 2341(2) Å 3 , Z = 4; polymorph B: SG = Cc, a = 53.451(2) Å, b = 4.707(1) Å, c = 9.178(1) Å, β = 91.53(7)°, V = 2308(2) Å 3 , Z = 4. Polymorph C has been detected in powder samples obtained by precipitation in aqueous solution. Its structure was solved from X-ray synchrotron diffraction data: SG = Pna2 1 , a = 9.3386(6) Å, b = 4.7393(3) Å, c = 54.646(5) Å, V = 2418.5(3) Å 3 and Z = 4. The three polymorphs have the usual layered structures encountered in zinc linear carboxylates series with Zn atoms in tetrahedral O-coordination. The forms A and C have the same building blocks but differ by their stacking. Two adjacent layers are 180° rotated around the short and the long axis in A and C respectively. The form B presents conformational disorder leading to modification of the preceding layers. In these form, the layers maintain the same orientation along the stacking direction. [ABSTRACT FROM AUTHOR]

Sodium borosilicate base glasses modeled on French nuclear waste materials were prepared to test the dependence of crystallization product quantity and distribution on cesium- and molybdenum-loading and glass cooling rate. Scanning electron microscopy shows the presence of micrometer-sized domains of Mo-rich crystalline precipitates. X-ray diffraction patterns are complex but reveal the presence of sodium molybdates and CsNaMoO4·2 H2 O. 133 Cs and 23 Na magic-angle spinning NMR spectroscopy exhibit distinct peaks for glassy and crystalline phases which can be quantified to obtain the identities of the individual compounds that are formed as well as the fractions of these nuclei in particular crystalline phases. In these model systems, 1 mol% Mo can be entirely incorporated into the glassy network whereas 2.5 and 5 mol% Mo produce significant quantities of crystalline phases, with little dependence on cooling rate. Cesium content appears to have a weak influence on crystallization behavior. Sodium molybdate and sodium-cesium molybdate hydrate are the dominant devitrification phases in all cases. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Coffinite, USiO4, is an important U(IV) mineral, but its thermodynamic properties are not well-constrained. In this work, two different coffinite samples were synthesized under hydrothermal conditions and purified from a mixture of products. The enthalpy of formation was obtained by high-temperature oxide melt solution calorimetry. Coffinite is energetically metastable with respect to a mixture of UO2 (uraninite) and SiO2 (quartz) by 25.6 ± 3.9 kJ/mol. Its standard enthalpy of formation from the elements at 25 °C is -1,970.0 ± 4.2 kJ/mol. Decomposition of the two samples was characterized by X-ray diffraction and by thermogravimetry and differential scanning calorimetry coupled with mass spectrometric analysis of evolved gases. Coffinite slowly decomposes to U3O8 and SiO2 starting around 450 °C in air and thus has poor thermal stability in the ambient environment. The energetic metastability explains why coffinite cannot be synthesized directly from uraninite and quartz but can be made by low-temperature precipitation in aqueous and hydrothermal environments. These thermochemical constraints are in accord with observations of the occurrence of coffinite in nature and are relevant to spent nuclear fuel corrosion. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 D4h13-P42/mbc of the tetragonal system. Their structure features infinite one-dimensional chains of ∞¹[An(PS4)22-] 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 Ak2+An4+(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 for SrU(PS4)2 and BaU(PS4)2 using the HSE 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. [ABSTRACT FROM AUTHOR]

Abstract: The barium uranium sulfides Ba3.69US6, BaUS3, and BaU2S5 have been synthesized at 1273, 1323, and 1323K, respectively. The structures of these compounds have been determined by single-crystal X-ray diffraction methods. They crystallize in space groups , , and , respectively. Among these three ternaries and Ba2US6 the coordination geometries about the U atoms vary considerably. Ba3.69US6 is a mixed U4+/U5+compound whereas the others are U4+ compounds. The decreasing dimensionality of the four compounds with increasing Ba–to–U content shows the expected trend. [Copyright &y& Elsevier]

Abstract: Crystal structures of boron-containing AFm (B-AFm) and AFt (B-AFt) phases have been solved ab-initio and refined from X-ray powder diffraction. 11B NMR and Raman spectroscopies confirm the boron local environment in both compounds: three-fold coordinated in B-AFm corresponding to HBO3 2− species, and four-fold coordinated in B-AFt corresponding to B (OH)4 − species. B-AFm crystallizes in the rhombohedral space group and has the 3CaO·Al2O3·CaHBO3·12H2O (4CaO·Al2O3·1/2B2O3·12.5H2O, C4AB1/2H12.5) general formulae with planar trigonal HBO3 2− anions weakly bonded at the centre of the interlayer region. One HBO3 2− anion is statistically distributed with two weakly bonded water molecules on the same crystallographic site. B-AFt crystallizes in the trigonal P3cl space group and has the 3CaO·Al2O3·Ca(OH)2·2Ca(B (OH)4)2·24H2O (6CaO·Al2O3·2B2O3·33H2O, C6AB2H33) general formulae with tetrahedral B (OH)4 − anions located in the channel region of the structure. All tetrahedral anions are oriented in a unique direction, leading to a hexagonal c lattice parameter about half that of ettringite. [Copyright &y& Elsevier]

Abstract: Decommissioning of old nuclear reactors may produce waste streams containing chlorides and carbonates, including radioactive 36Cl− and 14CO3 2−. Their insolubilization by calcium monosulfoaluminate hydrate was investigated. Carbonates were readily depleted from the solution, giving at thermodynamic equilibrium monocarboaluminate, monocarboaluminate+calcite, or calcite only, depending on the initial ratio between the anion and calcium monosulfoaluminate hydrate. Chloride ions reacted more slowly and were precipitated as Kuzel''s salt, Kuzel''s and Friedel''s salts, or Friedel''s salt only. Rietveld refinement of X-Ray powder diffraction patterns was successfully used to quantify the phase distributions, which were compared to thermodynamic calculations. Moreover, analysing the lattice parameters of Kuzel''s salt as a function of its chloride content showed the occurrence of a restricted solid solution towards the sulfate side with general formula 3CaO·Al2O3·xCaCl2·(1− x)CaSO4·(12−2x)·H2O (0.36≤ x ≤0.50). [Copyright &y& Elsevier]

Abstract: The (bis aqua)(sebacato)magnesium Mg((κ2)-(κ1-κ1)-μ3-C10H16O4)(H2O)2 was synthesized by hydrothermal route. The structure was solved ab initio using optimisation methods from powder X-ray diffraction (PXRD) laboratory data. The structure crystallises in the monoclinic space group P21/c with a = 17.336(1) Å, b = 7.456(1) Å, c = 9.573(1) Å, β = 96.43 (1) °, V = 1229.61(9) Å3, Dx = 1.407 g cm−3 and Z = 4. Rietveld refinement from 399 reflexions leads to a reliability factor R bragg = 0.06. The structure consists of double sheets of Mg2+ centred O-octahedra linked by H-bonds and bridged by sebacate anions along [100], building a 3D metal-organic framework ‘MOF’. Heating at 100 °C provokes the condensation of the octahedra in an anhydrous compound Mg(C10H16O4). [Copyright &y& Elsevier]

Abstract: The crystal structure of Kuzel''s salt has been successfully determined by synchrotron powder diffraction. It crystallizes in the rhombohedral R3̅ symmetry with a =5.7508 (2) Å, c =50.418 (3) Å, V=1444.04 (11) Å3. Joint Rietveld refinement was realized using three X-ray powder patterns recorded with a unique wavelength and three different sample-to-detector distances. Kuzel''s salt is the chloro-sulfoaluminate AFm phase and belongs to the layered double hydroxide (LDH) large family. Its structure is composed of positively charged main layer [Ca2Al(OH)6]+ and negatively charged interlayer [Cl0.50·(SO4)0.25·2.5H2O]−. Chloride and sulfate anions are ordered into two independent crystallographic sites and fill successive interlayer leading to the formation of a second-stage compound. The two kinds of interlayer have the compositions [Cl·2H2O]− and [(SO4)0.5·3H2O]−. The crystal structure explains why chloride and sulfate anions are not substituted and why the formation of extended solid solution in the chloro-sulfate AFm system does not occur. [Copyright &y& Elsevier]

Abstract: The metastable phase (phase 1) Zn(OH)2(tp)2 (tp=C8H4O4 2−) was found to be an intermediate forming during the hydrothermal synthesis of Zn3(OH)4tp (phase 2). Its structure has been determined ab initio from synchrotron powder diffraction data and refined with the Rietveld method: space group P21/c, a =3.48856(2)Å, b =5.84645(2)Å, c =22.1331(1)Å, β =103.46(1)°, D x =2.488g/cm3, R p =0.10, R B =0.095 (402 independent reflections). The structures of the two analogues were compared. Whereas a mixed coordination of the zinc atoms was found in phase 2, phase 1 exhibits only penta-coordinated Zn(II). Moreover, different optical properties were observed, Zn2(OH)2(tp) showing photoluminescence at 378nm under λ ex =316nm. [Copyright &y& Elsevier]

Abstract: The structure of the new hybrid compound [Ni3(OH)2(tp)2(H2O)4]·2H2O (tp=C8H4O4 2−) has been determined ab initio from synchrotron powder diffraction data and refined with the Rietveld method: space group P-1, a =10.2077(6)Å, b =8.0135(5)Å, c =6.3337(4)Å, α =97.70 (1)°, β =97.21(1)°, γ =108.77(1)°, Dx =2.124g/cm3, Rp =0.045, R B =0.095 (757 independent reflections). H atoms were placed geometrically and their position optimized by DFT calculation. The repeating structural unit is the chain [Ni(1)O6]2Ni(2)O6, consisting of two edges sharing octahedrons related by the symmetry center and linked via μ3-OH to a vertex of Ni(2) octahedron. The Ni(1) coordination is ensured by two oxygen atoms from two water molecules, two OH and two oxygen atoms from carboxylate groups. The linkage of the chains by the tp anions forms infinite layers parallel to the (010) planes. Interchain hydrogen bonds between the water molecules coordinating the metal ensure the cohesion of the 2D structure. The structural and magnetic properties are compared with that of the 3D fumarate-based compound [Ni3(OH)2(fum)2(H2O)4]·2H2O (fum=C4H2O4 2−). [Copyright &y& Elsevier]

Coffinite, USiO4, is an important U(IV) mineral, but its thermodynamic properties are not well-constrained. In this work, two different coffinite samples were synthesized under hydrothermal conditions and purified from a mixture of products. The enthalpy of formation was obtained by high-temperature oxide melt solution calorimetry. Coffinite is energetically metastable with respect to a mixture of UO2 (uraninite) and SiO2 (quartz) by 25.6 ± 3.9 kJ/mol. Its standard enthalpy of formation from the elements at 25 °C is -1,970.0 ± 4.2 kJ/mol. Decomposition of the two samples was characterized by X-ray diffraction and by thermogravimetry and differential scanning calorimetry coupled with mass spectrometric analysis of evolved gases. Coffinite slowly decomposes to U3O8 and SiO2 starting around 450 °C in air and thus has poor thermal stability in the ambient environment. The energetic metastability explains why coffinite cannot be synthesized directly from uraninite and quartz but can be made by low-temperature precipitation in aqueous and hydrothermal environments. These thermochemical constraints are in accord with observations of the occurrence of coffinite in nature and are relevant to spent nuclear fuel corrosion. [ABSTRACT FROM AUTHOR]

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 ß 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 mO·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. [ABSTRACT FROM AUTHOR]

Abstract: The unusual oxidation state +3 of the thorium has been stabilized into a lithium containing non-stoichiometric mixed-valence (III/IV) thorium fluorinated phase with formula Li2+x Th12F50 (0

The long-term durability of magnesium potassium phosphate cement (MKPC) pastes was investigated by examining their leaching behavior. MKPC comprised magnesium oxide (MgO) and potassium dihydrogen phosphate (KH 2 PO 4) in equimolar amounts and yielded K-struvite (MgKPO 4 ·6H 2 O) and a nearly neutral pore solution pH upon hydration. Semi-dynamic leaching tests were performed on MKPC paste samples using demineralized water with a pH set at 7, and the leached solids were analyzed using XRD, SEM/EDS, 11B and 31P MAS-NMR spectroscopy. Leaching was mainly governed by diffusion of dissolved species through the pore network of the paste. Three main zones were observed in the leached solids: (i) a poorly cohesive residual layer where K-struvite was fully depleted, (ii) an intermediate zone where K-struvite coexisted with cattiite (Mg 3 (PO 4) 2 ·22H 2 O), and (iii) a third zone without any cattiite. Reactive transport modeling made it possible to predict the extent of degradation and the phase evolution in the MKPC paste samples. • Magnesium phosphate cement paste was investigated under leaching conditions. • A zonation process occurred in the leached solid. • K-struvite dissolved and cattiite precipitated transiently. • A reactive transport model helped describing the paste evolution under leaching. [ABSTRACT FROM AUTHOR]

Abstract In this study, the potential of SiC-TiC nano-composites as solar absorbers has been studied. For solar thermal applications, materials with high solar absorptance and low emittance are ideally sought for (spectral selectivity). A semi-molecular sol-gel synthesis route leading to nanometric homogenous composites was described. The resulting SiC-TiC nanocomposite powder was sintered at different temperatures to produce samples with various relative densities (from 57% to 96%). The samples morphology and composition were characterized by several techniques including Scanning Electron Microscopy (SEM), Energy-Dispersive X-Ray Spectroscopy (EDX), X-Ray Diffraction (XRD), carbon and oxygen elemental analyses. The link between the surface roughness and the relative density was precised and the effects on the optical properties (0.25–25 µm wavelength range) were studied. Comparisons were made with pure SiC and pure TiC samples with various relative densities. Overall, the sample emittance was found to strongly decrease with the increase in the relative density, leading to a great increase in the spectral selectivity, despite a little decrease in the solar absorptance. The TiC-SiC composite has an intermediate reflectance compared to the pure SiC and the pure TiC samples. With an absorptance of 0.76, an emittance of 0.44 and a selectivity of 1.74, the denser SiC-TiC could be a good candidate for bulk solar applications. Highlights • Sol-gel synthesis route enable to produce homogenous SiC-TiC nanocomposite powder. • SiC-TiC nanocomposite materials have good optical properties (spectral selectivity). • Spectral selectivity increases with the relative density of the sintered material. • Spectral selectivity decreases with the increase in surface roughness. [ABSTRACT FROM AUTHOR]