CHEMICAL-TRANSPORT AND STRUCTURE OF UNB2O7 - A NEW-TYPE OF MM(2)'O7

Powdery UNb2O7 could be obtained by heating an aquimolar mixture of UO2 and Nb2O5 in an evacuated silica tube at 1100-degrees-C. Chemical transport in a small temperature gradient with NbCl5/Cl2 as transport agent was successfully used for crystal synthesis. UNb2O7 crystallizes with orthorhombic symmetry; the lattice constants are a = 3.8012(7) angstrom; b = 21.170(8) angstrom; c = 6.440(2) angstrom and Z = 4. Structure 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 is surrounded by oxygen like an doma-octahedron, niobium1 like an pentagonal-bipyramid and niobium2 like an distorted octahedron.