Crystal structure of a helix layered silicate containing tetramethylammonium ions in sodalite-like cages

The crystal structure of a helix layered silicate (HLS), [(CH3)(4)N](2)Na-2[Si10O20(OH)(4)].5,53H(2)O, with tetramethylammonium (TMA) ions as templates was determined by ab initio structure analysis with X-ray powder diffract ion data. The HLS is orthorhombic with space group Amm2 and lattice parameters of a = 22.8641(2) Angstrom, b = 12.5388(2) Angstrom, and c = 12.4648(2) Angstrom. A Q(3)-Q(4)-Q(3) silicon network with an amount-of-substance ratio of Si/O = 1:2.4 exhibits cup-shaped cage topology with four- and six-membered rings. Such a cage is similar to a sodalite cage divided into two pieces. Si-29 MAS NMR showed the silicate layer to have Q(3)- and Q(4)-types of Si atoms with a Q(3)/Q(4) ratio of ca. 4.0, which is consistent with our structural model. The framework is distorted: with Si-O bond lengths varying widely. TMA ions are incorporated into the cup-shaped cage. Na+ ions and H2O molecules are also located between two silicate layers, with interlayer distances varying alternately. Disordering of TMA ions, Na+ ions, and H2O molecules was clearly visualized in electron-density maps obtained by combining a maximum-entropy method and whole-pattern fitting. H2O molecules surround Na+ ions and form hydrogen bonds with O atoms in silanol groups in the silicate layers, by which the lattice instability due to the distorted framework is compensated. The present compound is regarded as a metastable phase, as would be expected for precursors to new types of microporous materials with silicate frameworks such as the sodalite cage.