Synthesis and crystal structure of Th2Ta6O19, the first example of a ''Jahnberg-structure'' with M=M(4+) (M=Th4+); With a note to LaNb7O19

Colorless, hexagonal crystals of Th2Ta6O19 were obtained by chemical transport in the temperature gradient 1 000 degrees C --> 980 degrees C using a mixture of ThO2, ZrO2 (or HfO2 and Ta2O5 (1 : 2 : 2) as starting material and Cl-2, ZrCl4, or HfCl4 as transport agents. The lattice constants are a = 6.275(1)Angstrom, c = 19.968(6) Angstrom and Z = 2. Structure determination in the space group P6(3)/mcm (no. 193) let to R(1) = 0.032 (wR(2) = 0.074). Thorium is surrounded by oxygen like an trans bis-capped octahedron (CN = 8) and tantalum like a pentagonal bipyramid (CN = 7). Both coordination polyhedra are for themself arranged to layers (Th to o-, Ta to p-layers) so that in the direction of the c-axes a sequence of layers like p-p-o-p-p-o appears. Therefore the compound is a new representative of the structures described by Jahnberg.