Author:
de Jong B. H. W. S.,Supèr H. T. J.,Spek A. L.,Veldman N.,Nachtegaal G.,Fischer J. C.
Abstract
Li2Si2O5,
M
r
= 150.05, λ(Mo Kα) = 0.71073 Å, 293 K, orthorhombic, Ccc2, a = 5.807 (2), b = 14.582 (7), c = 4.773 (3) Å, V = 404.2 (3) Å3, μ(Mo Kα) = 0.78 mm−1, Z = 4, D
x
= 2.466 Mg m−3, R
1 = 0.045 for 249 reflections with I > 2.0σ(I), wR
2 = 0.1138 for all 261 reflections. K2Si2O5, M
r
= 214.36, λ(Mo Kα) = 0.71073 Å, 150 K, monoclinic, Cc, a = 16.322 (2), b = 11.243 (2), c = 9.919 (1) Å, β = 115.97 (1)°, V = 1636.4 (4) Å3, μ(Mo Kα) = 2.11 mm−1, Z = 12, D
x
= 2.610 Mg m−3, R
1 = 0.0385 for 2370 reflections with I > 2.0σ(I), wR
2 = 0.0785 for all 2711 reflections. The crystal structures of Li2Si2O5 and K2Si2O5 have been determined and refined. The lithium phyllosilicate sheet topology and structure based on three-dimensional data and refined in the orthorhombic space group Ccc2 confirm the previous determination by Liebau [Acta Cryst. (1961), 14, 389–395] from two-dimensional data, but described in the lower monoclinic Cc space-group symmetry. Potassium disilicate, on the other hand, is not a phyllosilicate but forms a three-dimensional defect cristobalite structure built from Q
3 silica units. In this structure, one Si—O—Si bridge, common to four six-membered T-rings, i.e. rings consisting of six silica tetrahedra in the chair conformation, is missing, resulting in 14-membered T-rings. Continuous sheets built up from these rings form a layer connected to layers above and below by six-membered T-rings in the chair conformation. 29Si MASNMR shows a good correlation between Si—O—Si angle and 29Si chemical shift for K2Si2O5, Li2Si2O5 and KLiSi2O5. The 29Si spin lattice relaxation times, T
1, are 335, 689 and 〈1256〉 s for Li2Si2O5, KLiSi2O5 and K2Si2O5, respectively. The pronounced non-linearity in hygroscopicity in glassy potassium–lithium disilicates is not observed in the osmolality of 0.8M potassium–lithium chloride aqueous solutions.
Publisher
International Union of Crystallography (IUCr)
Subject
General Biochemistry, Genetics and Molecular Biology,General Medicine