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Li2Si2O5, Mr = 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, Dx = 2.466 Mg m−3, R1 = 0.045 for 249 reflections with I > 2.0σ(I), wR2 = 0.1138 for all 261 reflections. K2Si2O5, Mr = 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, Dx = 2.610 Mg m−3, R1 = 0.0385 for 2370 reflections with I > 2.0σ(I), wR2 = 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 Q3 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, T1, 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.
