New anion-excess, fluorite-related ordered structure: Bi2Te2W3O16

A solid-state investigation of the Bi2O3-TeO2-WO3 system allowed synthesis of a new stoichiometric compound, Bi-2 Te2W3O16 (MX(2.286)). Bi2Te2W3O16 crystallizes with monoclinic symmetry (space group C2/c) with the unit cell parameters a = 21.2871(8) Angstrom, b = 5.5708(3), c = 12.8349(5) Angstrom, beta = 124.08(3)degrees, and Z = 4 formula units per cell. The structure was determined from single-crystal X-ray diffraction data and refined to residuals R(1)(F) = 0.036 and wR(2)(F-2) = 0.089 for 106 variable parameters and a data set of 1756 observations [I > 2 sigma(I)]. The essential building units of the structure are BiO8 square antiprisms, TeO4 disphenoids, and WO6 octahedra. Pairs of BiO8 square antiprisms share one edge to form a Bi2O14 unit. Each Bi2O14 unit shares four corners with four identical Bi2O14 units, two edges and four corners with W(2)O-6 octahedra, and four edges with TeO4 disphenoids, so constituting [Bi-2 Te2W(2),O-16] complex layers parallel to (100). These layers are connected via W(1)O-6 to form the Bi2Te2W3O16 three-dimensional network. Its relationships with the fluorite structure and the fluorite-related Bi7O5F11 structure are evidenced and analyzed. (C) 1996 Academic Press.