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 (a = 5.8007(8) Å, b = 11.5811(15) Å, c = 23.157(3) Å, α = 89.709(10)°, β = 88.915(11)°, γ = 89.004(11)°, V = 1555.1(4) Å³, Z = 8, D_x = 2.615 g · cm^–3, μ(Mo‐K_α) = 7.94 cm^–1). A characteristic feature of the crystals is a twinning by reticular pseudo‐merohedry, which simulates a much larger monoclinic C centered lattice (V′ = 6220 Å³, Z = 32). The twin element corresponds to a twofold rotation axis running parallel to the 0 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, Ca₂MgSi₂O₇]) or AlO₄] tetrahedra (as in gehlenite, Ca₂AlAlSiO₇]). By sharing common edges, the NaO₄] tetrahedra in Na₆Si₂O₇ are forming columns running parallel to 25 . The resulting framework contains tunnels in which the more irregularly coordinated sodium cations are incorporated.