STRUCTURAL CHARACTERIZATION OF HIGH-PRESSURE C-NA2SI2O5 BY SINGLE-CRYSTAL DIFFRACTION AND 29SI MAS NMR

Abstract

Single crystals of C-Na2Si2O5 have been synthesized from the hydrothermal recrystallization of a glass. The title compound is monoclinic, space group P2(1)/c with Z = 8 and unit-cell parameters a = 4.8521 (4)Angstrom, b = 23.9793(16)Angstrom, c = 8.1410(6)Angstrom, beta = 90.15(1)degrees and V = 947.2(2)Angstrom(3). The structure has been determined by direct methods and belongs to the group of phyllosilicates. It is based on layers of tetrahedra with elliptically six-membered rings in chair conformation. The sequence of directedness within a single ring is UDUDUD. The sheets are parallel to (010) with linking sodium cations in five- and sixfold coordination. Concerning the shape and the conformation of the rings, C-Na2Si2O5 is closely related to beta-Na2Si2O5. However, both structures differ in the stacking sequences of the layers. A possible explanation for the frequently observed polysynthetic twinning of phase C is presented. In the Si-29 MAS-NMR spectrum of C-Na2Si2O5 four well-resolved lines of equal intensity are observed at -86.0, -86.3, -87.4, and -88.2 ppm. The narrow range of isotropic chemical shifts reflects the great similarity of the environments of the different Si sites. This lack of pronounced differences in geometry renders a reliable assignment of the resonance lines to the individual sites on the basis of known empiric correlations and geometrical features impossible.