The structure of K-hydrosodalite

Hydrosodalite, Na6[AlSiO4]6 · 8H2O, is a synthetic modification of natural sodalite with water molecules replacing for the anion at the centre of the β   cage. Hydrosodalite has a cubic structure with space group P4¯3n and a ≈ 8.9 Å. Cation ordering and framework distortions leading to symmetry reduction were observed for synthetic sodalites but have never been reported for hydrosodalite. This work reports the first experimental evidence of a symmetry reduction in hydrosodalite. The investigated sample is a K-exchanged hydrosodalite with chemical formula K6[AlSiO4]6 · 7.8H2O. The structure refinement was performed using synchrotron powder diffraction data in space group P1 with a = 9.196(1) Å, b = 9.188(1) Å, and c = 9.205(1) Å, α = 89.69(1)°, β = 90.43(1)°, and γ = 89.791(9)°. K+ strongly interacts with all the framework oxygen atoms of the hexagonal ring and provokes anisotropic distortion of the framework. In the distorted framework, the only possible distribution scheme for the K+ ions is ordered, to prevent any Coulomb repulsion between adjacent ions. In addition, the ordered scheme is the only to ensure even distribution of the positive cation charges with respect to the Al atoms (Al:K ratio is 1:1). In fact, each of the six Al atoms has three K+ ions at a distance of 3.5–3.9 Å and in turn, each of the six K+ ions has three Al atoms at a distance of 3.5–3.9 Å. K+ ions display a coordination number from 8 to 10 and mean K–O distance, including the water molecules, of 2.921–3.043 Å.