ON THE CRYSTAL-CHEMISTRY OF THE BARIUM OX OMETALLATE-TELLURITES BA(2)M(6)TE(2)O(21) (M EQUALS NIOBIUM AND TANTALUM)

Single crystals of Ba2Nb6Te2O21 (I) and Ba2Ta6Te2O21 (II) have been pr epared by solid state reactions in air. X-ray investigations led to mo noclinic symmetry, space group C-2h(3) -C2/m, (I): a=16.699(3), b=5.67 1(2), c=9.611(2) Angstrom, beta=96.92(2), (II): a=16.655(8) b=5.667(2) , c=9.576(4) Angstrom, beta=96.64(7), Z=2. Nb5+ or Ta5+ show octahedra l and Te4+ a one sided triangular coordination by O2-. The position an d distance of the lone pair on Te4+ have been estimated by calculation s of the Coulomb terms of the lattice energy. The polyhedron around Ba 2+ may be described by a triply capped BaO8 cube. The crystal structur es are dominated by a(infinity)(3)[Nb6O21](12-) network with incorpora ted Ba2+ and Te4+ ions.