CHEMICAL-TRANSPORT AND STRUCTURE OF UNB2O 7 - A NEW-TYPE OF MM(2)'O7

Powdery UNb2O7 could be obtained by heating an aquimolar mixture of UO 2 and Nb2O5 in an evacuated silica tube at 1100-degrees-C. Chemical tr ansport in a small temperature gradient with NbCl5/Cl2 as transport ag ent was successfully used for crystal synthesis. UNb2O7 crystallizes w ith orthorhombic symmetry; the lattice constants are a = 3.8012(7) ang strom; b = 21.170(8) angstrom; c = 6.440(2) angstrom and Z = 4. Struct ure determination in the space group Cmcm (nr. 63) let to R = 0.021 (R (w) = 0.019). U and Nb1 have CN = 7, Nb2 has CN = 6 (5 + 1). Uranium i s surrounded by oxygen like an doma-octahedron, niobium1 like an penta gonal-bipyramid and niobium2 like an distorted octahedron.