PREPARATION, CRYSTAL-STRUCTURE AND ELECTR ON-MICROSCOPIC INVESTIGATION OF UNB6O16 - A NEW NIOBIUM-RICH PHASE IN THE SYSTEM U NB/O/

Powdery UNb6O16 was produced by heating (1000-degrees-C or 1 100-degre es-C; evacuated silica tube) mixed powders of UO2, NbO2 and Nb2O5 (1:2 :2). Single-crystals of UNb6O16 were obtained by chemical transport in a small temperature gradient (1000-degrees-C-->990-degrees-C; transpo rt agent NH4Cl). The lattice constants are a = 22.339(4) angstrom; b = 3.7750(6) angstrom; c = 7.249(3) angstrom; beta = 97.61(3)-degrees an d Z = 2. The structure determination (space group C2) let to R = 0.026 (R(w) = 0.026). Eight oxygen atoms surround U4+ like a trans-bis-capp ed octahedron, Nb4+ and Nb5+ are coordinated distorted octahedraly. Th e structure was checked and the occupation of the positions O8 and O9 was clarified with the program MAPLE4 [3]. A through focus series of h igh resolution transmission electron microscopic images was obtained w hich is in acceptable agreement with images calculated on the basis of the multi-slice method.