ζ-Y2[Si2O7]: Ein neuer Strukturtyp in der Yttrialit-Reihe / ζ -Y2[Si2O7]: A New Structure Type within the Yttrialite Series

Abstract

Abstract During attempts of preparing yttrium oxotellurates(IV) using Y2O3 and TeO2 in YCl3 fluxes, the occasional reaction of these educts with the walls of the evacuated silica ampoules led to colourless, lath-shaped single crystals of Y2[Si2O7] in the new ζ -type structure as a minor by-product which was investigated by X-ray diffraction. The title compound crystallizes monoclinically in the space group P21/m (a = 503.59(5), b = 806.47(8), c = 732.65(7) pm, β = 108.633(6)°) with two formula units per unit cell. The crystallographically unique Y3+ cation is coordinated by seven oxygen atoms (d(Y-O = 221 - 248 pm) arranged in the shape of a slightly distorted monocapped octahedron. The isolated oxodisilicate units [Si2O7]6− consist of two Si4+ cations and seven O2− anions of which five are crystallographically independent. These pyroanions (d(Si-O) = 161 - 168 pm, ∢ (O-Si-O) = 91 - 117°, ∢ (Si-O-Si) = 156°) exhibit an almost perfectly eclipsed conformation built of a horseshoeshaped backbone with the two silicon and three of the oxygen atoms situated on the mirror planes of the unit cell. The remaining four oxide anions complete this [Si2O7]6− entity of two vertex-sharing [SiO4]4− tetrahedra as terminal ligands for silicon. Assembled in planar layers parallel to (−1 0 1), the [Si2O7]6− anions are packed with their wide basal faces of the tetrahedra pointing towards the small waist of the adjacent units and vice versa. The yttrium cations reside between these layers in order to interconnect them three-dimensionally.