Structural studies on Na6Si8O19-a monophyllosilicate with a new type of layered silicate anion

The existence of a sodium silicate with composition Na6Si8O19 has been known for more than forty years. However, until now the crystal structure of this phase was unknown. In the course of this work single crystals of the Na6Si8O19 have been prepared from the devitrification of a glass at ambient pressure. The title compound is monoclinic, space group P121/c1 with Z=4 and pseudo-orthorhombic unit-cell parameters: a=4.9038(5)Å, b=23.481(3)Å, c=15.392(2)Å, β=90.14(1)° and V=1772.3(4)Å3. The structure has been solved by direct methods and refined to a residual of R(|F|)=0.064 for 2327 independent observed reflections. Furthermore, micro-Raman spectra have been obtained from single crystal grains. Na6Si8O19 belongs to the group of phyllosilicates and is based on layers of tetrahedra parallel to (001). Individual sheets can be described as being built by the condensation of four different zweier single chains of SiO4-tetrahedra running parallel to the a-axis. Due to the specific linkage of the chains the single silicate layers consist of both threefold and fourfold connected tetrahedra in the ratio Q3:Q4=3:1. Stacking of the layers along to the c-axis results in a three dimensional structure in which the sodium cations reside on six crystallographically independent sites between the layers for charge compensation. They are coordinated by 5-6 oxygen ligands forming distorted trigonal bipyramids and octahedra, respectively.