SYNTHESIS AND STRUCTURE OF M(4)TA(18)O(53 ) (M=TH, U) - THE FIRST REPRESENTATIVES WITH A NEW STRUCTURE TYPE A-LA-JAHNBERG

Colorless [brownblack], hexagonal crystals of Th4Ta18O53 [U4Ta18O53] w ere obtained by chemical transport. Several mixtures of powders were u sed as starting materials for chemical transport reactions. Cl-2 or Ta Cl5/Cl-2 were applied as transport agents in several temperature gradi ents. The lattice constants are a = 6.2554(5) Angstrom (6.253(1) Angst rom), c = 27.709(5) Angstrom (27.609(6) Angstrom) and Z = 1. Structure determination in the space group P6(3)/mcm (no. 193) let to R1 = 0.02 69 (wR2 = 0.0629) (R1 = 0.0295 (wR2 = 0.0674)). Thorium as well as Ura n is surrounded by oxygen like an trans-bis-capped octahedron (CN = 8) and Tantalum1 and Tantalum2 like pentagonal-bipyramids (CN = 7). The formula (Th2O3)(2)(Ta3O8)(6) corresponds to the principle of building for the ideal construction. Because of necessarity of valence compensa tion the number of O-atoms in the real structure is diminished by one. The Th-O- and U-O-coordination polyhedra are for themselves arranged to o-layers and the Ta1-O- and Ta2-O-bipyramids for each other to p-la yers. These layers are edge linked along the c-axes with a sequence of p-p-p-o-p-p-p-o, In fact of this these compounds are the first exampl es of the new structure type p-p-p-o as a member of a structure-family first described by Jahnberg. The various members differ in the sequen ce of layers with octahedral (o) and pentagonal-bipyramids (p).