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Eggenstein-Leopoldshafen, Germany

Carvajal Nunez U.,European Commission | Eloirdi R.,European Commission | Prieur D.,European Commission | Martel L.,European Commission | And 4 more authors.
Journal of Alloys and Compounds

In this study, uranium dicarbide (UC2) has been prepared by arc melting and heat treated under vacuum to form uranium sequicarbide (U 2C3) in the presence of a second phase UC2 -z. Both samples, as cast and heat treated, have been characterised by chemical analyses, X-ray diffraction (XRD), 13C magic angle spinning nuclear magnetic resonance (MAS-NMR) and by extended X-ray absorption fine structure (EXAFS). The composition, the purity, the various environments of both U and C atoms as well as the bonds length with the coordination number have been determined. By combining a long-range order method (XRD) and short-range order spectroscopy techniques (EXAFS and NMR), a unique view on the microstructure of UC2, before and after heat treatment, and of U2C3 phase has been achieved. © 2013 Elsevier B.V. All rights reserved. Source

Carvajal Nunez U.,European Commission | Martel L.,European Commission | Prieur D.,European Commission | Lopez Honorato E.,CINVESTAV | And 4 more authors.
Inorganic Chemistry

A series of uranium carbide samples, prepared by arc melting with a C/U ratio ranging from 0.96 to 1.04, has been studied by X-ray diffraction (XRD), 13C nuclear magnetic resonance (NMR), and extended X-ray absorption fine structure (EXAFS). XRD determines phase uniqueness and the increase of the lattice parameter versus the carbon content. In contrast, 13C NMR detects the different carbon environments in the lattice and in this study, clearly identifies the presence of discrete peaks for carbon in the octahedral lattice site in UC and an additional peak associated with excess carbon in hyperstoichiometric samples. Two peaks associated with different levels of carbon deficiency are detected for all hypostoichiometric compositions. More than one carbon environment is always detected by 13C NMR. This exemplifies the difficulty in obtaining a perfect stoichiometric uranium monocarbide UC1.00. The 13C MAS spectra of uranium carbides exhibit the effects resulting from the carbon content on both the broadening of the peaks and on the Knight shift. An abrupt spectral change occurs between hypo- and hyperstoichiometric samples. The results obtained by EXAFS highlight subtle differences between the different stoichiometries, and in the hyperstoichiometric samples, the EXAFS results are consistent with the excess carbon atoms being in the tetrahedral interstitial position. © 2013 American Chemical Society. Source

Prieur D.,European Commission | Carvajal-Nunez U.,European Commission | Vitova T.,Institute For Nukleare Entsorgung Ine | Somers J.,European Commission
European Journal of Inorganic Chemistry

X-ray diffraction and X-ray absorption spectroscopy have shown that a material composed of two (Pu,Am)O2-x substoichiometric fluorite-type structures is obtained by sintering in moisturized Ar/H2. The Pu oxidation state is strictly +IV, whereas Am exhibits a +III/+IV mixed valence. Thus, the Am-O system and particularly the Am oxidation state play a major role in the sintering behavior of the Pu-Am-O system. The heating of this compound in air leads to the formation of a fluorite Pu0.80Am 0.20O1.98 solid solution. The transition from a one-phase to a two-phase structure takes place at a temperature lower than 800 °C. The results were compared with previous studies on Am-bearing UO2 and ThO2, which have evidenced different behaviors. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Source

Carvajal-Nunez U.,European Commission | Prieur D.,European Commission | Vitova T.,Institute For Nukleare Entsorgung Ine | Somers J.,European Commission
Inorganic Chemistry

The electronical and structural properties of Th 0.80Am 0.20O 2-x materials have been studied by the coupling of X-ray diffraction and X-ray absorption spectroscopy techniques. A substoichiometric fluorite Th IV 0.80Am III 0.20O 1.90 solid solution is found following sintering in moisturized Ar-H 2. In contrast, heating of this sample in air leads to a nondefective fluorite Th IV 0.80Am IV 0.20O 2.00 solid solution. The structures of these solid solution compounds were fully characterized by assessing the interatomic distances, the coordination numbers, and the structural disorder. The effect of the sintering atmosphere on these crystallographical parameters and on the cation valences has been determined and the capability of ThO 2 to accommodate tri- and tetravalent actinides in the fluorite structure assessed. © 2012 American Chemical Society. Source

Polly R.,Institute For Nukleare Entsorgung Ine | Schimmelpfennig B.,Institute For Nukleare Entsorgung Ine | Florsheimer M.,Wiesbaden University of Applied Sciences | Rabung T.,Institute For Nukleare Entsorgung Ine | And 3 more authors.
Radiochimica Acta

Sorption plays a major role in the safety assessment of nuclear waste disposal. In the present theoretical study we focused on understanding the interaction of trivalent lanthanides and actinides (La3+, Eu 3+ and Cm3+) with the corundum (110) surface. Optimization of the structures were carried out using density functional theory with different basis sets. Additionally, Møller-Plesset perturbation theory of second order was used for single point energy calculations. We studied the structure of different inner-sphere complexes depending on the surface deprotonation and the number of water molecules in the first coordination shell. The most likely structure of the inner-sphere complex (tri- or tetradentate) was predicted. For the calculations we used a cluster model for the surface. By deprotonating the cluster a chemical environment at elevated pH values was mimicked. Our calculations predict the highest stability for a tetradentate inner-sphere surface complexes with five water molecules remaining in the first coordination sphere of the metal ions. The formation of the inner-sphere complexes is favored when a coordination takes place with at most one deprotonated surface aluminol group located beneath the inner-sphere complex. The mutual interaction between sorbing metal ions at the surface is studied as well. The minimal possible distance between two inner-sphere sorbed metal ions at the surface was determined to be 530 pm. © by Oldenbourg Wissenschaftsverlag. Source

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