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Free keywords:
Single-layer silicate; Sodium disilicate; C-Na2Si2O5; Single-crystal structure analysis; Twinning; Phyllosilicate; 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 P21/c with Z = 8 and unit-cell parameters a=4.8521(4)Å, b=23.9793(16)Å, c=8.1410(6)Å, β=90.15(1)° and V=947.2(2)Å3. 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 β-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 29Si 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.
