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[H7O3]4[Tc20O68] ⋅ 4H2O [1] was prepared from an aqueous Tc2O7 solution concentrated over anhydrous H2SO4. [Tc20O68]4− is the first polyanionic species to be reported for Tc. The unit cell contains one centrosymmetric [Tc20O68]4− polyanion as well as hydronium ions and water molecules. The core of the structure consists of four Tc(V)O6 octahedra that form a square Tc4O4 ring. The four Tc(V)O6 octahedra are decorated by sixteen Tc(VII)O4 tetrahedra. Calculations show the bonding within the Tc4O4 ring to consist of a 3‐center bond formed between each neighboring pair of Tc atoms and their bridging oxygen. Calculations also indicate that a strong d→d electronic transition at 513 nm is the origin of the red color of [1]. The characterization of red HTcO4 solutions by X‐ray absorption spectroscopy has complemented the description of this compound in aqueous solution. The formation mechanisms in solution, including the possible role of technetium's radioactivity in the formation of [1], are discussed. [ABSTRACT FROM AUTHOR]

Among the fission products present in the spent nuclear fuel, technetium exhibits a singular behavior in reprocessing operations performed by solvent extraction. Indeed, this strong acid readily dissociates to form the oxo-anion TcO4− that may interfere with uranium(VI), plutonium(IV), and zirconium(IV) in the extraction cycles of the PUREX process. This paper focuses on the uranium-technetium complex with TBP and on its non-radioactive rhenium surrogate. Despite the large set of distribution data available for rhenium and technetium extraction with TBP, the structures of the co-extracted complexes remain largely unknown. However, it is important to understand clearly the extraction mechanism of technetium with TBP in the PUREX process to optimize the separation process and to model its behavior during the extraction steps. Based on distribution data available in the literature, a thermodynamic model was developed for the extraction of technetium with TBP for a large excess of uranium(VI) in organic phase. A good representation of uranium and technetium distribution data was thus obtained when considering the formation of (HTcO4)(TBP)n complexes, as well as mixed UO2(NO3)(TcO4)(HNO3)x(TBP)n, complexes. In the complex UO2(NO3)2(HNO3)x(TBP)n., one pertechnetate anion replaces one nitrate in the uranium coordination sphere. Combination of complementary spectroscopic techniques (FT-IR and X-ray absorption) supported by theoretical calculations (density functional theory) with organic phases containing a large excess of technetium(VII) or rhenium(VII) enabled full characterization of the limit mixed uranium−technetium species and also of mixed uranium-rhenium species. Details on the coordination of the uranium-technetium complex are provided with the help of DFT calculations and XAS measurements. [ABSTRACT FROM AUTHOR]

Separation processes based on room temperature ionic liquids (RTIL) and electrochemical refining are promising strategies for the recovery of lanthanides from primary ores and electronic waste. However, they require the speciation of dissolved elements to be known with accuracy. In the present study, Eu coordination and EuIII/EuII electrochemical behavior as a function of water content in 1‐ethyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIm][NTf2]) was investigated using UV–visible spectrophotometry, time‐resolved laser fluorescence spectroscopy, electrochemistry, and X‐ray absorption spectroscopy. In situ measurements were performed in spectroelectrochemical cells. Under anhydrous conditions, EuIII and EuII were complexed by NTf2, forming Eu−O and Eu−(N,O) bonds with the anion sulfoxide function and N atoms, respectively. This complexation resulted in a greater stability of EuII, and in quasi‐reversible oxidation–reduction with an E0' potential of 0.18 V versus the ferrocenium/ferrocene (Fc+/Fc) couple. Upon increasing water content, progressive incorporation of water in the EuIII coordination sphere occurred. This led to reversible oxidation–reduction reactions, but also to a decrease in stability of the +II oxidation state (E0'=−0.45 V vs. Fc+/Fc in RTIL containing 1300 mm water). [ABSTRACT FROM AUTHOR]

Although the preparation of CeSiO4 has been already reported, the formation of pure cerium silicate from aqueous precursors appears as a challenge. An innovative way of synthesis has been identified in this study, allowing the formation of CeSiO4 after hydrothermal treatment starting from Ce(iii) silicate precursors. Among the experimental parameters examined, significant effects were found according to the nature of the precursor and of the reactive media considered, the pH of the reactive media and the temperature of the hydrothermal process. This study allows the determination of optimized conditions for the hydrothermal synthesis of pure CeSiO4 (A-Ce2Si2O7 or Ce4.67(SiO4)3O as starting precursors, nitric medium, pH = 7, 7 days at 150 °C). The in situ low oxidation rate of Ce(iii) into Ce(iv) was a key parameter to consider in order to avoid the presence of CeO2 in the final mixtures. [ABSTRACT FROM AUTHOR]

A new polarizable force field for describing the solvation of the uranyl (UO22+) cation in solvent extraction phases has been developed for molecular dynamics simulations. The validity of the polarizable force field has been established by comparison with EXAFS and SWAXS experiments. This new force field allows for describing both the UO22+ hydration and solvation properties in good agreement with the experiments. In aqueous phases we demonstrated that the UO22+ force field has been improved from the previous one we developed. Indeed, the UO22+ structural and dynamics properties, i.e., the dynamics of the water molecules in the vicinity of the uranyl cation, calculated from molecular dynamics are in better agreement with the EXAFS experiments. Furthermore, the transferability of the UO22+ force field proposed here has been validated on typical solvent extraction phases containing uranyl nitrate salts with extractant molecules, namely DMDOHEMA molecules, in n-heptane. The good agreements observed between the theoretical (MD simulations) and experimental UO22+ structures at the molecular (EXAFS) and supramolecular (SWAXS) scales prove the accuracy of the UO22+ force field developed and proposed in the present paper. [ABSTRACT FROM AUTHOR]

In this paper, we prove the existence of extremal functions for the best constant of embedding from anisotropic space, allowing some of the Sobolev exponents to be equal to 1. We prove also that the extremal functions satisfy a partial differential equation involving the 1 Laplacian. [ABSTRACT FROM AUTHOR]

Three tetravalent actinide (AnIV) hexanuclear clusters with the octahedral core [An6(OH)4O4]12+ (AnIV=UIV, NpIV, PuIV) were structurally characterized in the solid state and in aqueous solution by using single-crystal X-ray diffraction, X-ray absorption, IR, Raman, and UV/Vis spectroscopy. The observed structure, [An6(OH)4O4(H2O)8(HDOTA)4] ⋅HCl/HNO3 ⋅ n H2O (An=U( I), Np( II), Pu( III)), consists of a AnIV hexanuclear pseudo-octahedral cluster stabilized by DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) ligands. The six actinide atoms are connected through alternate μ3-O2− and μ3-OH− groups. Extended X-ray absorption fine structure (EXAFS) investigations combined with UV/Vis spectroscopy provide evidence for the same local structure in moderate acidic and neutral aqueous solutions. The synthesis mechanism was partially elucidated and the main physicochemical properties (pH range stability, solubility, and protonation constant) of the cluster were determined. The results underline the importance of: 1) considering such polynuclear species in thermodynamic models, and 2) competing reactions between hydrolysis and complexation. It is interesting to note that the same synthesis route with thorium(IV) led to the formation of a dimer, Th2(H2O)10(H2DOTA)2 ⋅4 NO3 ⋅ x H2O ( IV), which contrasts to the structure of the other AnIV hexamers. [ABSTRACT FROM AUTHOR]

To enable the reduction of the environmental impact of nuclear energy generation, in this paper, we link the molecular and macroscopic behaviour of a functionalized material (TR@SBA15) used to extract uranium from sulfuric media. Two organic 3-[ N, N-di(2-ethylhexyl)carbamoyl]-3-[ethoxy(hydroxy)phosphoryl]propanoic acid (TR) molecules grafted onto the solid are involved in the extraction process and form a 2:1 TR-U complex. FTIR and extended X-ray absorption fine structure (EXAFS) spectroscopic analyses show that the TR-U bond is realized through a phosphonate group in a monodentate fashion below pH 3, in agreement with the macroscopic observations. The first coordination sphere of the uranyl ion is completed by two monodentate sulfate ions and one water molecule. Above pH 3, the TR contribution decreases, and other inner-sphere complexes appear, which is consistent with the increased extraction observed on the macroscopic scale. Molybdenum, a competitor element, reduces the uranium extraction capacity but not its speciation, whereas polyoxomolybdates form inside the pores from the molybdenum in solution. [ABSTRACT FROM AUTHOR]

In the context of the separation of uranium and plutonium from spent fuel allowed by N,N -dialkylamides, three regioisomers of N,N -di(2-ethylhexyl) butyramide (DEHBA or ββ) and the diastereopure isomers of N -(2-ethylhexyl)- N -(oct-3-yl)butyramide (EHOBA or αβ) were synthesized to assess their extraction performance and to study the mechanisms at the origin of the differences observed between the stereo- and regioisomers. The N,N -dialkylamides showed differences in extraction, with a greater effect of regio- than stereoisomerism. A mechanistic study at both the molecular and supramolecular scales was initially applied to explain these effects. X-ray absorption and UV-vis spectroscopy showed that uranium is extracted by a UO 2 (NO 3 ) 2 L 2 complex, which is not very sensitive to steric hindrance, while plutonium is extracted by two complexes, Pu(NO 3 ) 4 L 2 and Pu(NO 3 ) 6 (HL) 2 , which are differently affected by stereo- and regioisomerism. Investigations at the supramolecular scale also showed that Pu(NO 3 ) 4 L 2 complexes are disadvantaged by the bulkiness of the extractants, while Pu(NO 3 ) 6 (HL) 2 is favored by the preformation of larger supramolecular aggregates.

Actinide colloids and nanoparticles (NPs) currently constitute a topic of strong interest due to their potential role in advanced nuclear energetics and the environmental migration of radioactivity. A better understanding of the physico-chemical properties of nanoscale actinide oxides requires robust synthesis approaches. In this work, UO 2+ x NPs were successfully prepared by sonochemistry from U(IV) solutions previously stabilised in a hydrochloric medium (20 kHz, 65 °C, Ar/(10%)CO). Colloidal suspensions were found to be composed of crystalline and spherical NPs showing a UO 2 -like structure and measuring 18.0 ± 0.1 nm (SAXS, HR-TEM and PXRD techniques). In comparison with the controlled hydrolysis approach used as a reference, sonochemistry appears to be a simple and original synthesis route providing larger, better defined and more crystalline UO 2+ x NPs with a narrower size distribution. These well-defined NPs offer new opportunities for the preparation of reference actinide materials devoted to fundamental, technological and environmental studies.

New insights are provided about the formation mechanism of PuO 2 nanoparticles (NPs) by investigating an unprecedented kinetic isotope effect observed during their hydrolytic synthesis in H 2 O or D 2 O and attributed to OH/OD zero point energy difference. The signature of a Pu(IV) oxo-hydroxo hexanuclear cluster, appearing as an important intermediate during the formation of the 2 nm PuO 2 NPs (synchrotron SAXS/XAS), is further revealed indicating that their formation is controlled by H-transfer reactions occurring during hydroxo to oxo-bridge conversions.

Due to the increased attention given to actinide nanomaterials, the question of their structure-property relationship is on the spotlight of recent publications. Plutonium oxide (PuO 2 ) particularly plays a central role in nuclear energetics and a comprehensive knowledge about its properties when nanosizing is of paramount interest to understand its behaviour in environmental migration schemes but also for the development of advanced nuclear energy systems underway. The element plutonium further stimulates the curiosity of scientists due to the unique physical and chemical properties it exhibits around the periodic table. PuO 2 crystallizes in the fluorite structure of the face-centered cubic system for which the properties can be significantly affected when shrinking. Identifying the formation mechanism of PuO 2 nanoparticles, their related atomic, electronic and crystalline structures, and their reactivity in addition to their nanoscale properties, appears to be a fascinating and challenging ongoing topic, whose recent advances are discussed in this review., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

In this study, we propose to use classical molecular dynamics (MD) coupled with 1 H NMR spectroscopy to study the conformations of different actinyl An VI (An = U, Np, and Pu) and An V (An = Np) complexes with tetra-ethyl dyglicolamide (TEDGA) ligands in order to have a better representation of such complexes in solution. Molecular dynamics simulations showed its effectiveness in interpreting the experiments by the calculation of geometric factors needed for the determination of magnetic properties of these complexes. We demonstrated that different conformations of the An V and An VI complexes with TEDGA exist in solution with different coordination modes, which is experimentally confirmed by 1 H NMR and EXAFS spectroscopies. Furthermore, MD simulations provide additional insights into the structures of complexes in solution since conformations with fast exchanges, which are not accessible from NMR experiments, have been observed by MD simulations.