JRP CALL information
Supported By

European Commission

Short description of the work
During this stay, the work done at HZDR-IRE was dedicated to the synthesis and the characterization of new neptunium-based coordination polymers, involving new neptunium crystal chemistry. The synthesis with 4,4’-biphenyldicaboxilic acid (H2-bpdc) in dimethylformamide (DMF) leaded to the crystallization of very nice octahedral crystals. Crystal data were collected and analyzed during this stay at HZDR-IRE. The corresponding structure revealed for the first time the isolation of the hexanuclear cluster An6O8 with neptunium in a solid state. During this stay, the same inorganic building unit was also stabilized with other carboxylate-based ligands (terephthalate and 2,6-naphtalendicarboxylate). The structural determination of these two solids is still in progress. We also analyzed the stabilization of new neptunium based clusters, from oxygen-free aqueous solutions with phthalic and mellitic acids as organic ligand precursors. We isolated two new compounds, showing also new neptunium crystal chemistries (infinite chains and layers). Based on the obtained data, several publications are expected and the manuscript will be prepared by early 2016.

 

Short description of the work
The samples described in the proposal could not have been measured in the framework of this call as an unexpected problem preventing the sample preparation occurred 2 weeks before the beamtime. For this reason, we measured several compounds in the Bi-U-O ternary system in the frame of the safety assessment of lead bismuth eutectic (LBE) cooled fast reactors.
Cubic fluorite-type phases have been reported in the UIVO2-Bi2O3 system for the entire compositional range, but a non-linear evolution of the lattice parameter with uranium substitution has been observed. In the current extensive investigation of the uranium(IV) oxide – bismuth (III) oxide system, the unusual behaviour of the lattice parameter evolution with composition has been confirmed and its origin identified. Even under inert atmosphere at 800 °C, UIV oxidises to UV/UVI as a function of the substitution degree. Thus, using a combination of three methods (XRD, XANES and Raman) we have identified the formation of the BiUVO4 and Bi2UVIO6 compounds, within this series..

 

Short description of the work
The speciation of uranium (VI) in aqueous media in presence of sulfates is complex with the coexistence of various species according to the sulfate concentration and the pH of the medium. In acidic conditions, UO2(SO4)(aq), UO2(SO4)22- and UO2(SO4)34- may co-exist. At pH > 3, the speciation is even more complex as oligomeric species or ternary species may form as a consequence of the uranium hydrolysis. Therefore, it is a challenge to perform an accurate characterization of the speciation in view to determine not only the stoichiometry of the species in solution, but also their structure. Sulfate ions have the ability to bind uranium (VI) in two ways, as monodentate or as bidentate fashion, and some contradictory results were obtained, especially at low sulfate ratio. According to authors, a monodentate or a bidentate coordination is evidenced for the 1:1 complex. In this context, we would like to use the specific properties of ionic liquids (IL) to investigate the coordination of sulfate ions with uranium (VI). In this kind of solvent constituted only by ions and easy to dry, we can get rid of the uranium (VI) hydrolysis. Also, we can choose the dissolved uranyl and sulfate salts so that their counter-ions are identical to the ions constituting the IL. In that way, we avoid any interferences due to counter-ions, the studied system being composed only by uranyl, sulfate and the cation and anion of the IL.
We used this experiment at the uranium L3 edge to identify the stoechiometry and configuration of sulfato-complexes of uranium(VI) in ionic liquids, as a function of the sulfate/uranium ratio in solution. We show in particular the formation of a solid compound which corresponds to the 1:1 complex where the sulfate group is bound to uranium in a bidentate fashion.

 

Short description of the work
In the framework of the partitioning and transmutation, the Am recycling is one of the milestones requiring notably a full understanding of the Am electronical properties. Different XANES studies at LIII edges have shown different charge distribution in uranium-americium mixed oxides. U(IV) and Am(III/IV) were found in sol-gel synthesized materials while U(IV/V) and Am(III) were observed in powder metallurgy prepared compounds. In order to understand this discrepancy, the U and Am oxidations states in uranium-americium oxides have been studied using high resolution XANES (HRXANES) at M4 edge. Compared to L3 XANES, M4 HR XANES allows a significant improvement of the spectrum resolution by overcoming the core-hole lifetime broadening ffects leading then to sharper spectral features. In this context, the M4 XANES spectra of U reference compounds (UO2, U3O8, UO3), Am reference compounds (AmO2) and uranium-americium mixed oxides have been collected. Comparing these latter spectra with the U and Am reference samples, it has been established that Am is purely trivalent in uranium-americium mixed oxides while U is tetra- and pentavalent.

 
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