Cationic ordering in tysonite type structures: II: Crystal structure of α-, β- and γ-baryum hexafluoridouranates (IV)

Abstract

Three polymorphic varieties of BaUF6 have been synthesized and their crystal structure determined by X-ray diffraction on single crystals.The high temperature α- BaUF6 belongs to the disordered tysonite type and its structure is compared to BaThF6 using the same defect models: ideal structure, split-model structure and introduction of anharmonic thermal vibration factor for cations. Both structures are similar.β-BaUF6 is stable in a narrow temperature range (962–992K) below the disordered variety. It crystallizes in the triclinic P-1 space group and contains 3 Ba and 3U independent cationic sites. The structure derives from tysonite type with BaF10 and BaF11 Edshammar polyhedra, similar to those described in tysonite, forming complex sheets alternating with homologous sheets of UF8 and UF9 polyhedra.The low-temperature γ-BaUF6 (T<962K) is a monoclinic P21/c ordered superstructure of the tysonite type, much simpler than β-BaUF6, with BaF11 Edshammar polyhedra and UF8 dodecahedra forming alternate zigzag layers.The structural features of both ordered superstructures allow to propose a model of short range order for all the disordered tysonite M(II)M’(IV)F6 phases. This model involves a decrease of the coordination number of M’(IV) cations and of the connectivity of M(II) and M’(IV) polyhedra from face-sharing to edge-sharing and from edge-sharing to corner-sharing, locally distorting the hexagonal sublattice.