Eric Simoni

Structural investigation and electronic properties of the nickel ferrite NiFe2O4: a periodic density functional theory approach

Periodic density functional theory (DFT) calculations using plane-wave basis sets were performed in order to study the bulk of nickel ferrite NiFe2O4. The local spin density approximation (LSDA) and the generalized gradient approximation (GGA) formalism were used, and it appeared that the LSDA failed to describe the magnetic structure of this compound. However, the GGA formalism gave reliable results in good agreement with experimental data for the lattice parameters, the electronic properties and the bulk modulus. In addition, the calculated density of states of the metallic species d block as well as their local magnetic moments were correlated to the crystal-field theory. Then, a charge deformation map was computed and, as expected from the electronegativity scale, the electron excess is localized around oxygen atoms along the bond axes. The formation energies of metallic vacancies are in good agreement with the inverse spinel structure experimentally observed.

Grazing incidence XAFS spectroscopy of uranyl sorbed onto TiO2 rutile surfaces

The surface complex formed by uranyl oxocations sorbed onto rutile titanium oxide has been probed by X-ray Absorption Spectroscopy. These measurements are part of a work that aims to model the interaction between heavy metal ions and mineral surfaces in aqueous conditions. In order to define the orientation and structure of the oxocation complex on the surface, both polycrystalline and monocrystalline (110) and (001) planes of rutile TiO2 have been investigated. Polarized XANES measurements show that the uranyl rod sorbs nearly parallel to the rutile surface, although this ideal configuration must be modulated by the surface topographic defects. Site by site simulation compared to the EXAFS data suggests that two surface oxygen sites are involved: shared edge and shared summit oxygen atoms of the TiO6 octahedron. Both grazing incidence EXAFS on the (110) plane and isotropic EXAFS on polycrystalline TiO2 reveal comparable sorption behavior: on average, the uranyl oxocation bonds to the surface as a bidentate complex with two short oxygen distances at 2.32 A and three larger distances at 2.47 A. Grazing incidence EXAFS on the (001) plane shows an unexpected low signal to noise ratio due to the lower uranium uptake. Data analysis suggests the formation of an outer sphere uranium complex on this plane. The discrepancy between both plane reactivities is still not understood.

Solving the hydration structure of the heaviest actinide aqua ion known: the californium(III) case.

The solution chemistry of actinide ions has been a fundamental question since the beginning of the nuclear technologies, given that the solvent stabilizes the high oxidation states of actinides. The development of procedures to avoid the migration of actinides from the already accumulated nuclear waste into natural water systems is a field of great activity. One of the primary properties of actinide ions in solution is their solvation, as it is intimately joined to complexation, precipitation, and resolution processes. The rareness and hazardousness of the heavier actinide elements, which steeply increase with the atomic number, has prevented a complete examination of the trends along the series, beyond the middle of the series. The curium cation Cm has often been considered as the heaviest actinide species characterized, and it has attracted much attention from both experimental and theoretical views in recent years. Systematic studies of the aqueous trivalent lanthanides have revealed a contraction of the metal–oxygen distance and a decrease of the total first coordination number along the series. Recent investigations using extended X-ray absorption fine structure (EXAFS) techniques have examined if this contraction takes place in a monotone or an irregular way along the series. The data available for the actinide series up to Cm indicates a similar contraction, 5, 7] although a conclusive answer cannot be given owing to the uncertainty of the structural data, particularly concerning the hydration number, and the scarce information on the second half of the series. Beyond the middle of the series, there is only one study reported for berkelium (Bk) and a preliminary EXAFS study for californium(III) carried out by one of us. Owing to the position of Cf in the actinide series, an accurate enough determination of the coordination number and Cf O distance could certainly shed light on the question of the actinide contraction. This objective gives the study a more fundamental than applied character, owing to the extreme rareness of this element. The most similar available crystallographic data of Cf with Cf O bonds is that of single crystals of Cf(IO3)3, which present a significantly distorted tricapped trigonal prism with a wide range of Cf O distances (2.353–2.921 ). This limited information does not meet the required level of accuracy for answering the question on the basis of a conventional EXAFS data analysis. Herein we present an alternative way to study this extreme case, by coupling new highly refined EXAFS data obtained in an actinide-dedicated beamline in the European Synchrotron Radiation Facility (ESRF, Grenoble), with the first Monte Carlo (MC) simulations of Cf in water. Specifically developed Cf OH2 intermolecular potentials based on ab initio quantum mechanical (QM) potential energy surfaces and the polarizable and flexible MCDHO water model have been used. Figure 1 shows the experimental and fitted k-weighted EXAFS spectra of a Cf aqueous solution using two model structures, the square antiprism configuration (SA; see Figure 2a), which represents an octacoordination of water

Ions in solutions: Determining their polarizabilities from first-principles

Dipole polarizabilities of a series of ions in aqueous solutions are computed from first-principles. The procedure is based on the study of the linear response of the maximally localized Wannier functions to an applied external field, within density functional theory. For most monoatomic cations (Li(+), Na(+), K(+), Rb(+), Mg(2+), Ca(2+) and Sr(2+)) the computed polarizabilities are the same as in the gas phase. For Cs(+) and a series of anions (F(-), Cl(-), Br(-) and I(-)), environmental effects are observed, which reduce the polarizabilities in aqueous solutions with respect to their gas phase values. The polarizabilities of H((aq)) (+), OH((aq)) (-) have also been determined along an ab initio molecular dynamics simulation. We observe that the polarizability of a molecule instantaneously switches upon proton transfer events. Finally, we also computed the polarizability tensor in the case of a strongly anisotropic molecular ion, UO(2) (2+). The results of these calculations will be useful in building interaction potentials that include polarization effects.

Analysis of absorption and luminescence spectra of U3+ doped Cs2NaYCl6 and Cs2LiYCl6 single crystals

Uranium(3+) doped single crystals of Cs2NaYCl6 and Cs2LiYCl6 with a 2.0% and 0.1% U3+ concentration have been obtained by the Bridgman-Stockbarger method. Luminescence spectra of the crystals were recorded at 160, 70, and 15 K. The emission bands observed in the visible and near infrared regions have been assigned to transitions from the lowest components of the 4I11/2, 4F3/2, and 4G7/2 multiplets to the crystal-field components of the 4I9/2 ground level. Absorption spectra were recorded from 4 000 to 25 000 cm−1 at 4.2 K. The zero phonon transitions were identified from an analysis of the vibronic side bands as well as in least-squares fits by applying a semiempirical Hamiltonian representing the combined atomic and crystal-field interactions for an ion of the 5f3 configuration and Oh symmetry. Eight energy level parameters were varied simultaneously in the least-squares adjustments yielding a mean error of 57 and 61 cm−1 for U3+:Cs2LiYCl6 and U3+:Cs2NaYCl6, respectively. The analysis of the spectra enab...

Periodic Density Functional Theory Investigation of the Uranyl Ion Sorption on Three Mineral Surfaces: A Comparative Study

Canister integrity and radionuclides retention is of prime importance for assessing the long term safety of nuclear waste stored in engineered geologic depositories. A comparative investigation of the interaction of uranyl ion with three different mineral surfaces has thus been undertaken in order to point out the influence of surface composition on the adsorption mechanism(s). Periodic DFT calculations using plane waves basis sets with the GGA formalism were performed on the TiO2(110), Al(OH)3(001) and Ni(111) surfaces. This study has clearly shown that three parameters play an important role in the uranyl adsorption mechanism: the solvent (H2O) distribution at the interface, the nature of the adsorption site and finally, the surface atoms’ protonation state.

Uranyl interaction with the hydrated (001) basal face of gibbsite: A combined theoretical and spectroscopic study

The sorption of uranyl cations and water molecules on the basal (001) face of gibbsite was studied by combining vibrational and fluorescence spectroscopies together with density functional theory (DFT) computations. Both the calculated and experimental values of O-H bond lengths for the gibbsite bulk are in good agreement. In the second part, water sorption with this surface was studied to take into account the influence of hydration with respect to the uranyl adsorption. The computed water configurations agreed with previously published molecular dynamics studies. The uranyl adsorption in acidic media was followed by time-resolved laser-induced fluorescence spectroscopy and Raman spectrometry measurements. The existence of only one kind of adsorption site for the uranyl cation was then indicated in good agreement with the DFT calculations. The computation of the uranyl adsorption has been performed by means of a bidentate interaction with two surface oxygen atoms. The optimized structures displayed strong hydrogen bonds between the surface and the -yl oxygen of uranyl. The uranium-surface bond strength depends on the protonation state of the surface oxygen atoms. The calculated U-O(surface) bond lengths range between 2.1-2.2 and 2.6-2.7 A for the nonprotonated and protonated surface O atoms, respectively.

Theoretical Investigation of the Uranyl Ion Sorption on the Rutile TiO2(110) Face

Canister integrity and radionuclide retention is of first importance for assessing the long-term safety of nuclear waste stored in engineered geologic depositories. Uranyl ion sorption on the TiO(2) rutile (110) face is investigated using periodic density functional theory (DFT) calculations. From experimental observations, only two uranyl surface complexes are observed and characterized. When the pH increases (from 1.5 to 4.5), the relative ratios of these two surface complexes are modified. From a crystallographic point of view, three sorption sites can be considered and have been studied with different protonation states of the surface to account for very acidic and low acidic conditions. The two surface complexes experimentally observed were calculated as the most stable ones, while the evolution of their sorption energies agrees with experimental data.

Theoretical chemical contribution to the simulation of the LIII X-ray absorption edges of uranyl, neptunyl and osmyl hydrates and hydroxides

XANES spectroscopy has long been used as a structural and electronic probe of a selected element. Phenomenological application of this technique to actinide cations has proved fruitful to characterize the actinide environment in both solid state and solution compounds. Although powerful XANES simulation codes have been developed, the use of such simulations in order to describe the valence orbitals of the actinide cation is still scarce. The very short life time of the core hole at the LIII edge as well as the low symmetry and large size of the coordination polyhedron are difficulties to be overcome in the analysis of the edge spectra. In this work, three simple molecules have been selected for their similar geometry that is typical of the trans dioxo actinyl compounds: [UO2(H2O)5]2+, [NpO2(H2O)5]2+, [NpO2(OH)4]2−. Additional comparison with a transition metal, the osmyl cation [OsO2(OH)4]2−, is also made. The cation LIII edges have been recorded and compared to edge calculations using FEFF8.2 code. This article is structured in two parts. In the first one, elaboration and optimization of a valid structural model cluster is carried out using molecular dynamics calculations. The influence of the water solvent molecules as well as the hydrogen atoms of the cations’ first coordination sphere are discussed. In the second part, Amsterdam quantum chemical calculations have been carried out on the four clusters and molecular energy levels are qualitatively compared to the data obtained from calculated XANES spectra.

Evolution of the uranium local environment during alteration of SON68 glass

Abstract The speciation of uranium in SON68 glass specimens doped with 0.75–3.5 wt% uranium and in the gels formed by alteration of the specimens was investigated by X-ray absorption spectroscopy. In the glasses, uranium is present at oxidation state VI and coordination number 6 with the same average distances than those found in a UO3 type environment. The U–O distances and uranium coordination numbers are identical throughout the uranium concentration range. During glass alteration the uranium remains at oxidation state VI in the gels, but was found in the uranyl form. An increase in the equatorial distances (from 2.20 and 2.32 A in the glass to respectively 2.22 and 2.39 A in the gel) and coordination numbers (to about 7 and 8, respectively) was observed.