Structure of KLiSi₂O₅ and the hygroscopicity of glassy mixed alkali disilicates

KLiSi₂O₅, M_r = 182.21, λ(Mo Kα) = 0.71073 Å, 150 K, monoclinic, P2₁, a = 5.9803 (6), b = 4.7996 (6), c = 8.1599 (11) Å, β = 93.477 (10)°, V = 233.78 (5) ų, μ(MoKα) = 15.7 cm⁻¹, Z = 2, F(000) = 180, D_x = 2.5885 (6) Mg m⁻³, R₁ = 0.0331 for 1023 reflections with I > 2.0σ(I), wR₂ = 0.0864 for all 1064 reflections. The crystal structure of potassium lithium phyllosilicate has been determined and the hygroscopicity of glassy single and mixed alkali disilicates has been measured. The potassium lithium phyllosilicate sheet topology is the same as that of lithium phyllosilicate, consisting of six-membered silica rings in a chair configuration. Li is fourfold oxygen coordinated by three non-bridging O and one bridging O, while K is sixfold oxygen coordinated by five non-bridging O and one bridging O. The K atoms are sandwiched between the LiSi₂O₅ layer. Crystalline and glassy lithium disilicate are not hygroscopic. Crystalline and glassy potassium disilicate are the most hygroscopic alkali disilicates known to date, whereas the mixed system, crystalline and glassy lithium potassium disilicate, is again not hygroscopic. Calculated valences for lithium potassium phyllosilicate do not provide a rationale for these observations.

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