|Description du poste :
|Mesoscopic organization of nuclear oxides. The atomic structure of a solid is generally studied by X-ray, electron or neutron diffraction followed by crystallographic analysis. Typically, this involves determining crystal symmetry, lattice constants, and atomic positions based on the position and intensity of Bragg peaks. However, classical methods of crystallographic analysis assume perfect periodicity of structure, whereas real materials are often imperfect, containing considerable deviations from a perfect lattice structure. An alternative to crystallographic analysis is the atomic pair distribution function (PDF) analysis method which is widely used for the structural study of non-crystalline materials . In this approach, not only Bragg diffraction intensities but also diffuse scattering intensities are included in the analysis. The experimental configuration used is identical to that of the classic X-ray or neutron powder diffraction experiment, except that the choice of the diffusion probe must be such that the diffraction vector q can be sufficiently large. The structure factor, S(q), is obtained by appropriately normalizing the diffracted intensity and applying standard corrections for sample environment, absorption, multiple scattering and inelastic scattering. However, if the structure is perfectly periodic, the use of PDF presents no advantage over conventional crystallographic analysis. The true power of this method is revealed when the structure is not perfectly periodic . In classical crystallographic analysis, deviations from periodicity are taken into account by thermal shift factors (Debye-Waller factors), a direct consequence of limiting the experimental data to Bragg peak intensities only. On the other hand, not only Bragg peaks but also diffuse scattering intensities are all included in the PDF modeling. This allows deviations from the ideal crystal to be modeled much more clearly. The object of the internship is the modeling of the experimental PDF of U3O8. We believe that this system is sensitive to disorder due to the combined effects due to non-stoichiometry and possible ferroelastic transformations . U3O8 is a key byproduct during the processing of yellowcake, a uranium concentrate powder obtained by leaching, and is important for the manufacture of nuclear fuel. It is also a product of the oxidation of nuclear fuel irradiated in a reactor following the accidental exposure of uranium dioxide pellets to atmospheric oxygen. In this particular context, understanding the organization at the mesoscopic scale can improve our knowledge base used to describe oxidation mechanisms.