Preparation, crystal structure and electron microscopic investigation of UNb6O16 - a new niobium-rich phase in the system U/Nb/O

Powdery UNb6O16 was produced by heating (1 000 C or 1 100 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 (1 000 C→990 C; transport agent NH4Cl). The lattice constants are a=22.339(4) A; b=3.7750(6) A; c=7.249(3) A; β=97.61(3) and Z=2. The structure determination (space group C2) let to r=0.026 (Rw=0.026). Eight oxygen atoms surround U4+ like a trans-bis-capped octahedron, Nb4+ and Nb5+ are coordinated distorted octahedraly. The structure was checked and the occupation of the positions O8 and O9 was clarified with the program MAPLE4. A through focus series of high resolution transmission electron microscopic images was obtained which is in acceptable agreement with images calculated on the basis of the multi-slice method. (orig.)