Chemical bond complexity in delta-phases (fluorite-related ternary oxides).

International audienceOxides with generic stoichiometry A_7O_12 occupy, in a generic phase diagram, an intermediate place between fluorite and bixbyite structure types. Their structure is derived from the ideal fluorite structure, and it is characterized by a more or less pronounced ordering of the O vacancies within the average fluorite sublattice. We believe that ternary oxides with this kind of formula provide interesting degrees of flexibility for understanding the structural characteristic of these structures in the context of fission product stabilization in nuclear fuels and in the field of actinide waste forms as well. We would like to discuss the structural characteristics of the chemical bonds in ternary systems consisting of trivalent and hexavalent cations. Eventually, in compounds with generic formula A{III}_6B{VI}O12, the A cation can be either an actinide or a rare earth element, whereas the B cation can be either a transition metal (W, Mo) or hexavalent uranium. We believe that studying the polyhedron of the sevenfold coordinated A ion is particularly interesting: it is often described in the literature as a mono-capped trigonal prism, with bond lengths exhibiting an extremely large dispersion, ranging for instance from 2.19 to 2.70 Å in Y6WO12. We would discuss the implications of this large dispersion of distances on the chemical bond characteristics, compare DFT models with experiments on pristine and irradiated specimens, and discuss the competitive metastability of the oxygen deficient cubic fluorite phase that can be observed in some of these systems, as for instance Y6MoO12