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Na 6 Si 2 O 7 – The Missing Structural Link among Alkali Pyrosilicates
Author(s) -
Kahlenberg Volker,
Langreiter Thomas,
Arroyabe Erik
Publication year - 2010
Publication title -
zeitschrift für anorganische und allgemeine chemie
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.354
H-Index - 66
eISSN - 1521-3749
pISSN - 0044-2313
DOI - 10.1002/zaac.201000120
Subject(s) - triclinic crystal system , crystallography , monoclinic crystal system , tetrahedron , tetragonal crystal system , alkali metal , crystal structure , crystal twinning , chemistry , lattice (music) , group (periodic table) , physics , microstructure , organic chemistry , acoustics
The crystal structure of sodium pyrosilicate (Na 6 Si 2 O 7 ) 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 $\rm\overline1$ ( 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) Å 3 , 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 Å 3 , Z = 32). The twin element corresponds to a twofold rotation axis running parallel to the [0 $\rm[0\overline21]$ direction of the triclinic cell. The compound belongs to the group of sorosilicates, i.e. it is based on [Si 2 O 7 ] 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 Na 6 Si 2 O 7 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 [NaO 4 ] tetrahedron. The resulting structural motif is similar to the one that can be observed in melilites, where linkage between the T 2 O 7 ( T : Al, Si) moieties is provided by [MgO 4 ]‐ (as in akermanite, Ca 2 Mg[Si 2 O 7 ]) or [AlO 4 ] tetrahedra (as in gehlenite, Ca 2 Al[AlSiO 7 ]). By sharing common edges, the [NaO 4 ] tetrahedra in Na 6 Si 2 O 7 are forming columns running parallel to 100. The resulting framework contains tunnels in which the more irregularly coordinated sodium cations are incorporated.