Na6Si2O7 - The Missing Structural Link among Alkali Pyrosilicates

The crystal structure of sodium pyrosilicate (Na6Si2O7) was solved from single crystal diffraction data and refined to an R index of 0.051 for 17034 independent reflections. The compound is triclinic with space group P (1) over bar (a = 5.8007(8) angstrom, b = 11.5811(15) angstrom, c = 23.157(3) angstrom, alpha = 89.709(10)degrees, beta = 88.915(11)degrees, gamma = 89.004(11)degrees, V = 1555.1(4) angstrom(3), Z = 8, D-x = 2.615 g.cm(-3), mu(Mo-K-alpha) = 7.94 cm(-1)). A characteristic feature of the crystals is a twinning by reticular pseudomerohedry, which simulates a much larger monoclinic C centered lattice (V' = 6220 angstrom(3), Z = 32). The twin element corresponds to a twofold rotation axis running parallel to the [0 (2) over bar1] direction of the triclinic cell. The compound belongs to the group of sorosilicates, i.e. it is based on [Si2O7] groups, which are arranged in layers parallel to (100). Charge compensation within the structure is accomplished by monovalent sodium cations distributed among 24 crystallographically independent positions. They are coordinated by four to six nearest oxygen neighbors. Most of the coordination polyhedra can be approximately described as distorted tetrahedra or tetragonal pyramids. An alternative understanding of Na6Si2O7 can be gained if the tetrahedrally coordinated sodium atoms are considered for the construction of a framework. Actually, each four of the dimers within a single slice are linked by a more or less distorted [NaO4] tetrahedron. The resulting structural motif is similar to the one that can be observed in melilites, where linkage between the T2O7 (T: Al, Si) moieties is provided by [MgO4](as in akermanite, Ca2Mg[Si2O7]) or [AlO4] tetrahedra (as in gehlenite, Ca2Al[AlSiO7]). By sharing common edges, the [NaO4] tetrahedra in Na6Si2O7 are forming columns running parallel to [100 T]. The resulting framework contains tunnels in which the more irregularly coordinated sodium cations are incorporated.