In the context of research on U/minor actinides for nuclear fuel reprocessing in the transmutation process, developments are first studied with surrogates containing uranium and lanthanides to facilitate testing. The tests consist of precipitating and calcining a hydrazinium uranium/cerium oxalate. The structure of this oxalate had not been previously determined, but was necessary to validate the physicochemical mechanisms involved.The present study, firstly demonstrates the structural similarity of the U/Ce oxalate phase (N2H5, H)(2.9)U1.1Ce0.9(C2O4)(5)center dot 10H(2)O, synthesized using a vortex precipitator for continuous synthesis of actinide oxalates, with previously known oxalates, crystallizing in P6(3)/mmc symmetry, obtained by more classical methods.This fast precipitation process induces massive nucleation of fine powders. Their structural and microstructural determination confirms that the raw and dried phases belong to the same structural family as (NH4)(2)U-2(C2O4)(5)center dot 0.7H(2)O whose structure was described by Chapelet-Arab in P6(3)/mmc symmetry, using single crystal data. However, they present an extended disorder inside the tunnels of the structure, even after drying at 100 degrees C, between water and hydrazinium ions. This disorder is directly related to the fast vortex method.This structure determination can be used as a basis for further semi-quantitative analysis on the U/minor actinides products formed under various experimental conditions. (C) 2014 Elsevier Inc. All rights reserved.

Dense and tailored porosity U1−yAmyO2−x (y=0.10; 0.15) pellets were fabricated to determine the influence of the microstructure on the self-irradiation effect at room temperature due to Am α decay. A first positron annihilation lifetime spectroscopy (PALS) approach showed the presence of vacancy type defects due to the cumulative decay dose. When He fills these vacancies, a void growth and swelling can occur. In addition, accurate diameter measurements with time were performed showing that a macroscopic swelling of 1% could be attained for the high density material. The stage value seems to depend on both type and porosity level. Swelling laws describing the diameter variation have been determined.

The CEA is currently assessing minor actinides (MA) recycling in nuclear fuels in fast neutrons reactors (FNR). Two routes are investigated: homogeneous recycling, where MA are added up to several percents in UPu-type fuel to be used in the whole core, and heterogeneous recycling, which consists in higher amounts of MA in uranium oxide fuel used in the periphery of the core. The studies include various subject areas: neutronics, thermo-physical properties, coolant type (sodium, lead, bismuth, helium), etc ..., and experimental work concerning the fabrication of minor actinides compounds.The first part of this work consisted in the structural study by XRD o AmO2. We studied the lattice parameter change versus time due to alpha self-irradiation. In this context, AmO2 powder was prepared in the ATALANTE facility in the CEA Marcoule. It is a first approach of swelling phenomenon at the structural scale which has been extended at the macroscopic scale in the second part of the work with the study of the geometrical stability in time of UO2 sintered pellets containing 30% Am. Pellets here fabricated by conventional powder metallurgy process in hot cell at the laboratory scale. Swelling behaviour has been studied by accurate diameter measurements performed along the pellets and correlated to the cumulative alpha decay dose. Up to 2.1 x 10(17) a decay in the sample, no evidence of pellet macroscopic swelling was observed. (C) 2010 Elsevier B.V. All rights reserved.

Structural transformations of Muscovite at temperature up to 1095 °C were determined using powder X-ray and neutron diffraction. Data were collected at room temperature from preliminary heated and quenched samples at 650 °C, 980 °C and 1095 °C. X-ray data were interpreted by either Rietveld method and neutron data, which complete the structural information by a better assignation of oxygen positions. With neutron data atom position was refined by fitting Pair Distribution Functions. It was found to be a progressive but continuous microstructural change, with the formation of an increasingly disorganized structure, but the layered organization of muscovite is maintained up to 1095 °C. Rietveld refinements from X-ray confirm the 6 to 5 coordination of Al atoms above 650 °C. It induces some structural changes as the orientation and mutual position of tetrahedrons in silicate layers. Pair Distribution Function refinements show the weakening of the long range structural organization, above 5 Å. At lower distance, a local order is maintained and the preferential alignments of both alumina unit pairs and silica tetrahedron were observed. This residual structural order of high-temperature muscovite is favorable to the achievement of textured ceramics.

In the U–Ce–O system, a solid solution (U,Ce)O2+x of fluorite type containing anionic excess is known in a wide composition range. For high values of x, it transforms to a (U1−yCey)4O9−δ phase deriving from the β-U4O9−δ type [ordered anion-excess fluorite superstructure phase; I-43d space group; a=21.7484(1)Å for y=0.10]. The crystal structure of (U0.9Ce0.1)4O9−δ has been refined by the Rietveld method on a powder sample measured on D2B at ILL Grenoble. The structural model, proposed by Bevan et al. for β–U4O9−δ and not fully confirmed till now, has been verified. The structure is based on an ordered distribution of cuboctahedral clusters U6O37 inside a fluorite matrix. A preferential ordering of Ce4+ (and U4+) on the so-called “centaur polyhedra” with 10 coordination is proposed, on the basis of bond valence calculations. The structure so determined has the composition M64O143 (MO2.234) and no traces of excess anions, completing the supposed composition up to M4O9, could be detected.

The direct reaction of hydrogen on a mixture KF, CaH2 and CaF2 in stoichiometric proportions allowed to synthesize a series of discrete phases KCaH3−xFx with x=1, 1.5, 2 and 2.5. All phases were indexed with an orthorhombic unit cell [e.g.: a=623.47(5), b=881.05(8), c=620.83(6) pm for KCaHF2]. Their crystal structures are isotypic with KCaF3 and have been refined by a X-ray Rietveld study. They derive from perovskite type by a Pm-3m→Pnma distortion. Contrary to the homologous NaMgH2F in which H− and F− anions are equally distributed in a disordered way in the two anionic sites 4c and 8d, in the KCaH3−xFx series, the hydride ions preferentially occupy the 8d site and begin to occupy the 4c site only for the two compositions richer in hydrogen.

The crystal structure of Ba58Ga22F180O is established by means of X-ray single crystal diffraction. It is tetragonal: a = 22.033(1) Angstrom, c = 17.626(1) Angstrom, Z = 2. The structure is solved in the space group 14/mmm (ndegrees139), using 3219 independent reflections. It is mainly built from a deficient arrangement of fluorite-type [FBa4] tetrahedra connected by edges and vertices which constitutes the skeleton of the structure, giving rise to large cavities in which lie isolated fluorine ions in tetrahedral and octahedral barium environment, isolated [F2Ba6] bitetrahedra, isolated barium ions in eight-coordination of fluorine and a complex arrangement of isolated [GaF6] octahedra and isolated [Ga2F10O] bioctahedra.

The crystal structure of a new tellurium IV oxyfluoride: KTe3O6F in the series MTe3O6F (M = K, Rb) has been determined on the basis of X-ray single crystal data (rhombohedral symmetry, R-3 space group, a = 948.60(10), c = 1377.4(2) pm, Z = 6, refinement program: SHELXL-97, R-1 = 0.037). The structure is based on twisted Te3O6F sheets, separated by K layers, in which TeO4F square pyramids share O corners and edges, forming hexagonal rings of the same nature as in hexagonal bronzes. The F anions are weakly bonded to Te cations (Te-F = 245.3 pm) and form quasi-independent KF units inserted inside a TeO2 layer structure. The E lone pair is stereochemically active.The O/F order in this phase and in other Te IV oxyfluorides is discussed. (C) 2002 Editions scientifiques et medicales Elsevier SAS. All rights reserved.

The cesium uranates Cs2UO4, Cs2U2O7, Cs4U5O17 and Cs2U4O12 were studied using X-ray Diffraction (XRD), neutron diffraction, X-ray Photoelectron Spectroscopy (XPS) and X-ray Absorption Spectroscopy (XAS) in an attempt to couple the crystallographic structure to the uranium valence state using the local uranium environment. The diffraction spectra were used for Rietveld refinement to determine the atomic positions and interatomic distances. These distances were subsequently used in Bond Valence Sum (BVS) calculations to determine the uranium valences. The XAS spectra give direct information on the local uranium environment regarding the U–O distances and the arrangement of the oxygen atoms around the central uranium. The difference between the monovalent uranates and the multivalent Cs2U4O12 is clearly established in all spectra, as well as in the crystal structures. The different valences present can be assigned to individual uranium lattice sites, but some amount of disorder is required to balance the charges.

The properties of calcium phosphates of biological interest depend strongly on their calcium/phosphorus atomic ratios (Ca/P). Therefore, the precision for the determination of this ratio is crucial. We have investigated the accuracy of quantitative X-ray diffractometry (QXRD) analysis for the determination of Ca/P in compounds with Ca/P ranging from 1.5 to 2. For 1.5 < Ca/P < 1.667, a high accuracy is obtained with relative uncertainties on Ca/P values between 0.1% and 0.4%. In this domain of composition, QXRD is more accurate than classical wet methods. For 1.667 < Ca/P < 2, the precision is lower and wet chemical methods appear to be superior.

The phases LnO1−xF1+2x, derived from the fluorite-type structure by anionic excess, form, for medium size Ln cations (Ln=Nd, Sm, Eu, Gd), a homogeneous series of orthorhombic symmetry (space group Pmmn, a≈b≈aF√2/2;c=aF). Structural comparison of these phases by X-ray powder diffraction allows us to make a better determination of their defect structure: O/F order in sheets perpendicular to the [001] axis and formation of 1:0:2 clusters which can be associated in dimer units. For Nd, an orthorhombic–tetragonal transition is explained by a change from 1:0:2 to denser 1:0:3 clusters. Comparison of the structural features of these phases with “vernier” modulated phases, isolated for Sm–Lu cations, allows a general classification of LnO1−xF1+2x phases at low temperature to be proposed. All are characterized by the preservation of an anionic O/F order in sheets perpendicular to the [001]F axis and by a progressive loss of freedom in the accommodation of the anionic excess (nature and orientation of the clusters, tendency to long-range ordering) from La to Lu, caused by the decrease in Ln size and therefore in Ln coordination.

After a short review of the main results of our activities in the field of fluorinated compounds, a comparison of the crystal structures of the MxTey(IV)Fz compounds is developed.

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Enthalpy of formation of UFeO4 at 298K is determined by high temperature solution calorimetry. The thermal effects of dissolution of UFeO4, (U3O4 Fe3O4) in liquid 2PbO.B2O3 are measured in separate experiments by dropping samples held at room temperature into liquid 2PbO.B2O3 maintained at 988K. The thermal effects of dissolution of these samples at infinite dilution in liquid 2PbO.B2O3 are derived from these measurements and the enthalpy of formation of UFeO4 at 298K is computed. The value obtained is : DeltaHdegrees(f,298K)(UFeO4) =-25.2+/-40.6 kJ/mol. Yet, the heat capacity of UFeO4 has been deduced from measurements of enthalpy increments in the temperature range of 473 to 1050 K and is represented by the following relation :473 - 1053K c(p)(UFeO4) = 127.32 + 0.037 T(K) - 1079130.4/T(K)(2) J/mol. K These experimental measurements will be included in Thermocalc database to perform a assessment of U-Fe-O ternary phase diagram.

The direct reaction of hydrogen on a mixture of Na + Mg or NaF + Mg allowed for synthesis of NaMgH3 and NaMgH2F, respectively. Both phases were indexed with an orthorhombic unit cell with dimensions a = 546.34, b = 770.30, c = 541.08 pm for NaMgH3 and a = 547.59, b = 769.68, c = 540.31 pm for NaMgH2F. The crystal structures were refined by X-ray Rietveld refinement. They derive from the perovskite structure type by a Pmm → Pnma distortion. In the hydridofluoride, H− and F− anions are equally distributed in the two anionic sites in a disordered way.

In order to establish the whole ternary phase diagram (U-Fe-O), which is one of the main systems in the nuclear safety and enrichment programs, the UFeO4 ternary compound must be characterized with more precision. Indeed, the knowledge of this compound is essential because an experimental determination of the isothermal section at 1300 K shows that UFeO4 is involved in three-phase equilibria. The purpose of this work is to obtain thermodynamic data and new structural information on this ternary oxide.

TeOF2 crystallizes in the monoclinic system (space group P21) with unit cell parameters a=551.3(1)pm, b=828.9(1)pm, c=530.7(1)pm, β=96.22(2)°, and Z=4. Its crystal structure was solved and refined to RB=0.058 and Rwp=0.116 on the basis of a Rietveld analysis of its X-ray powder pattern. Each tellurium atom is surrounded by six anions [4+2] (two oxygen, two fluorine and two other fluorine atoms at much longer distances). Because of the strong stereochemical activity of their lone pairs E, the coordination polyhedra can be described as distorted trigonal bipyramids TeO2F2E. These polyhedra associated by sharing O corners, form quasi-independent helical chains coiling along b. Te–F weak bonds, connecting the chains, give a 3D character to the structure. Structural relationships with α-TeO2 have been found and analysed.

During a reinvestigation of the Bi2O3-BiF3 system, a nonstoichiometric oxidefluoride, of composition range BI(F, O)(2.50)-Bi(F, O)(2.43) at 500 degrees C, was characterized by annealing at temperatures higher than 300 degrees C.Its crystal structure was solved by X-ray diffraction on a single crystal of composition Bi(F, O)(2.45) in the R-3m space group with cell parameters: a = 4.1378(9) Angstrom and c = 20.321(3) Angstrom (Shelxl program: wR(2) = 7.2%, R-1 = 3.5%),The structure of BiF1.90O0.55 derives from the well-known rhombohedral LnFO type by formation of 1:0:3 (or 1:0:2) clusters orientated along the [001] axis of the hexagonal associated cell in a partly ordered way. This orientation of the clusters preserves the O/F long range order characteristic of the LnFO type, despite a statistical replacement of almost half the O anions by F ones. As in the tetragonal anion-excess LaF1+2xO1-x and the orthorhombic SmF1+2xO1-x fluorite-related phases, the clustering mainly affects the F site: F anionic vacancies are associated to F-i interstitial anions and to F-r anions relaxed from the normal F site. In the clusters, Bi cations are in 9-fold or 10-fold coordination and the higher limit of anionic insertion (Bi2F4O composition) corresponds to a complete juxtaposition of 1:0:3 clusters, Near this limit, the excess anions tend to form quasi-continuous irregular 3(6) sheets perpendicular to the [001] axis of the hexagonal cell. (C) 1999 Academic Press.

BaTeF6 crystallizes with orthorhombic symmetry (space group Fdd2) and the unit-cell parameters: a=1505.9(3) pm, b=1520.6(3) pm, c=900.5(2) pm, Z=16. Its structure was solved and refined with SHELXL93 to R1=0.030 and wR2=0.077 on the basis of 1429 independent reflections recorded from a single crystal with an automatic 4-circle diffractometer. The Te(IV) atoms are 5-fold coordinate and their lone pair E is stereochemically active. The Ba atoms are 11-fold coordinate. This structure derives from the tysonite type (LaF3) by a cationic ordering between BaF11 Edshammar polyhedra and TeF5+6 polyhedra resulting from the distortion of similar Edshammar polyhedra under the influence of the stereochemically active lone pair of Te4+.

α-Ba2ZrF8 is prepared as systematically twinned crystals by hydrothermal synthesis (200°C) or as fine powders either on a sand bath (60°C) (aqueous HF medium) or by solid state reaction at 450°C. Synchrotron radiation was used because of ambiguities in indexations from conventional X-ray (pseudo-hexagonal symmetry). The structure was determined ab initio from synchrotron powder data. Neutron data were used for improving accuracy because some degree of non-stoichiometry was suspected. The cell is orthorhombic, space group Prima, Z = 4, with a = 9.7401(1) Å, b = 5.6147(1) Å and c = 11.8871(1) Å (synchrotron data, 25°C). The final neutron Rietveld refinement led to RP = 8.4 % and RB = 3.5 % for the stoichiometric Ba2ZrF8 formulation (sand bath sample). The structure is built up from [ZrF8] bicapped trigonal prisms isolated in a kinked fluorite matrix. The isostructural Pb2ZrF8 compound prepared in solution is also examined. An unexpected relationship with NaBaZrF7 is discussed.