Synthese et structure de trois niobouranates d'ions monovalents: TlNb 2U 2O 11,5, KNbUO 6, et RbNbUO 6
TlNb 2U 2O 11,5, KNbUO 6, and RbNbUO 6 single crystals were synthesized from melts of U 3O 8, Nb 2O 5, and monovalent carbonates. TlNb 2U 2O 11,5 is orthorhombic Pmnb with a = 7.713(3), b = 10.329(4), c = 13.947(5) Å, Z = 4. The structure was refined to R = 0.067 and Rw = 0.085 for 1840 independent reflections. RbNbUO 6 and KNbUO 6 are isotypic and orthorhombic Pcab with a = 7.627(2), b = 11.420(4), c = 15.739(5) Å for RbNbUO 6 and a = 7.579(2), b = 11.321(4), c = 15.259(5) Å for KNbUO 6 with Z = 8. The structure of KNbUO 6 was refined to R = 0.082 and Rw = 0.092 for 1325 independant reflections. The study shows that the niobouranates of monovalent ions constitute a new family where UO 7 and NbO 5 (or NbO 6) polyhedra are sharing edges and corners. Two structural types may occur: the first with polyhedra joined in such a way that tunnels are formed in which the monovalent ion is located (Tl in TlNb 2U 2O 11,5); the second with polyhedra linked in slabs, with the large cations between the slabs (K or Rb in KNbUO 6 or RbNbUO 6). In both cases, U 6+ forms a collinear uranyl group normal to the plane of five secondary bonds.