[en] UV absorption of β-ThBr₄: presents a plain absorption front and for the same temperature the threshold energy has the same value than the threshold energy of excitation function and photocurrent peak. Emission intensity and lifetime decrease when temperature increases from 300 K to 400K (extinction temperature). Results are interpreted either by molecular orbital levels of the ThBr₈^4- cluster or either by conduction and valence bands of the matrix above. Absorption and emission spectra of U⁴⁺ in α-ThBr₄ (where U⁴⁺ has a S₄ symmetry) between 300 K and 4.2 K allow indexation of 30 levels. Spectroscopic parameters are calculated in D_2d and S₄ symmetry. Comparison of these parameters with those of U⁴⁺ in β-ThBr₄ and β-ThCl₄ shows that crystal field force is practically the same in the three matrices but the structure transformation from β to α has more influence on B_q^k than ligand change from Br^- to Cl^-. Owing to very low phonon energy, fluorescence spectra of U⁴⁺ is easy to observe in α-ThBr₄ as it is in β-ThBr₄ and ThCl₄

[en] The aim of this work is to understand the comparative structural and intrinsic luminescence properties of the pure matrices α and β - ThBr₄, and to study the electronic structure by optical spectroscopy of the U⁴⁺ ion in the α-ThBr₄ matrix. 1)Under U.V. excitation, βThBr₄ is intensively fluorescent in the blue-purple and α-ThBr₄ is fluorescent in the red. The main results concerning β-ThBr₄ are the following: -the optical absorption in the U.V. is under the form of a sudden absorption front and for a same temperature, its threshold energy has the same value as the threshold energy of the excitation function and of the photocurrent peak; -the intensity and the life time of the emission decrease when the temperature increases from 300 K until 400 K ( extinction temperature). All the obtained results have been explained either with the molecular orbitals levels of the ThBr₈^4- cluster or with the valence and conduction bands of the pure matrix. 2)The absorption and emission spectra of U⁴⁺ in α-ThBr₄ (in which U⁴⁺ has a point symmetry S₄) obtained between 300 K and 4.2 K have allowed to index 30 levels. The calculation of the spectroscopic parameters F^k, ξ and B^k_q has been carried out in symmetry D_2d and S₄. The comparison of these parameters with those calculated for U⁴⁺ in β-ThBr₄ and β-ThCl₄ show that the global force of the crystalline field is practically the same in the three matrices, but that the structure transformation β→α occurs more on the values of these B^k_q than on the change of the ligands Br^-→Cl^-. On the other hand, it has been possible with the α-ThBr₄ matrix, or the β-ThBr₄ and the β-ThCl₄, to observe the fluorescence spectra of the U⁴⁺ ion (particularly weak phonons energies). (O.M.)

[en] Prediction of radionuclides migration through a geosphere is of fundamental interest in order to evaluate the safety of an underground radwastes repository. The authors have studied uranyl and europium (III) ion sorption mechanisms onto three phosphate compounds: ZrP₂O₇, Zr₂O(PO₄)₂, and Th₄(PO₄)₄P₂O₇. A spectroscopic investigation which has been previously reported has allowed one to determine all species involved in the sorption processes under study. This paper presents the modeling of experimental retention data using FITEQLv3.2 code (constant capacitance model). All sorption isotherms were successfully simulated with respect to the structural constraints. The authors have shown that uranyl sorption onto the solids under study is less influenced by electrostatic interactions between aqueous species and charged on the surface than is that for europium(III) ion. Because many experimental constraints, obtained from independent spectroscopic techniques, have been taken into account for the fitting procedure, the sorption constant values can be determined accurately

[en] Dealing with actinide elements in molecular chemistry may result in particularly attractive and exotic physico-chemical properties. In solution, one of the spectroscopic tools able to selectively probe the structural or electronic properties of these molecules is the X-ray absorption process. Different aspects of absorption edge or EXAFS analysis related to actinide studies are presented, including phenomenological and semi-quantitative approaches. (authors)

[en] The processes at the solid/solution interfaces lead generally to the retention/migration of radionuclides. In order to quantitatively estimate these phenomena and obtain the corresponding thermodynamic constants, it is necessary to model the retention experimental data. Nevertheless, in order to simulate these data at best, in parallel to the macroscopic studies, it is indispensable to characterize these interactions at the molecular scale by a structural approach of the interface (carried out with different spectroscopic techniques). Indeed, by this approach, in using the sorbed cation as structural probe, it is possible to define experimentally the different components occurring in sorption equilibria. In order to illustrate this methodology, several systems (substrate/radionuclide) will be presented. (O.M.)

[en] In order to determine the radionuclides' sorption constants on solid natural minerals, both thermodynamic and structural investigations, using spectroscopic techniques, are presented. The natural clays, that could be used as engineering barriers in the nuclear waste geological repository, are rather complex minerals. Therefore, in order to understand how these natural materials retain the radionuclides, it is necessary first to perform these studies on simple substrates such as single crystal, oxides and silicates, and then extrapolate the obtained results on the natural minerals. We examine in this paper the sorption processes of the hexavalent uranium on zircon (ZrSiO₄) and the trivalent curium on a natural clay (bentonite). The corresponding sorption curves are simulated using the results obtained with the following spectroscopic techniques: laser induced spectro-fluorimetry, X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (EXAFS), diffuse reflectance infrared Fourier transform (DRIFT). (authors)

[en] The low temperature form α-ThBr₄ has a tetragonal structure I4₁/a (C⁶_4h) in which the thorium ion is at a site of S₄ symmetry. Assuming that the ³H₄ ground state in this new form remains Γ₄ as in the β-ThBr₄ form, the polarized absorption spectrum at 4.2 K shows that D_2d is a good approximation. At the same time that the lines are sharp, a peculiarity of the absorption of U⁴⁺ when excited by white radiation is the existence of seven groups of two or four fluorescences with the same energy difference for each group. Four ³H₄ ground state Stark levels and their assignments have been determined from high resolution absorption Zeeman effect studies at different temperatures, assuming electric dipole transition selection rules for D_2d symmetry: Γ₅ at 105 cm^-1, Γ₁ at 468 cm^-1, Γ₁ at 618 cm^-1 and Γ₅ at 825 cm^-1. (orig.)

[en] We studied the physical and spectroscopic properties of ThBr₄ and ThCl₄. We made first a synthesis and studied the crystal growth to determinate the Uranium concentration in the crystals. This work allows an interpretation of the optical properties of an ion embedded in an incommensurate structure

No abstract available

[en] The magnetic properties of the cubic Ba₂MNpO₆ (M=Fe, Mn, Zn) compounds have been investigated by magnetic susceptibility and Moessbauer spectroscopy measurements. Several kinds of magnetic behaviour were observed: ferromagnetic with Ba₂FeNpO₆; antiferromagnetic with Ba₂MnNpO₆; paramagnetic with Ba₂ZnNpO₆. (author)