Journal article

Publication year: 2000

Pages: 2299-2306

Journal: Journal of Inorganic and General Chemistry

Volume number: 626

Issue number: 11

ISSN: 0044-2313

eISSN: 1521-3749

Publisher: Wiley

Abstract: 
Black crystals of U2Ta6O19 with hexagonal shape were obtained (at T-1) by chemical transport using HCl (p (HCl, 298 K) = 1 atm; silica tube) as transport agent in a temperature gradient (T-2 --> T-1; 1000 degreesC --> 950 degreesC) and using a mixture of UO2, Ta2O5, and HfO2 (or ZrO2) (1:2:2) as starting materials (at Ttl For the structure determination the best result was achieved in space group P6(3)/mcm (No. 193, a = 6.26(2) Angstrom, c = 19.86(6) Angstrom). U2Ta6O19 is isotypical to Th2Ta6O19. Tn the crystal structure each uranium atom is surrounded by oxygen atoms like a bi-capped trigonal antiprism and tantalum atoms like a pentagonal bipyramid (CN = 7). Like the "Jahnberg Structures" both coordination polyhedra arrange themselves in separate layers (U-O-polyhedra, in o-, Ta-O-polyhedra in p-layers) so that in the direction of the c-axis the sequence of layers is p-p-o. Using chemical transport it was possible to prepare the compounds Th12Nb16O63Cl2 and Th8M4Nb16O63Cl2 (M = Zr, Hf), which are the first quaternary and quinquinary examples in these systems. They crystallize isotypically.