Ab initio structure determination of novel small-pore metal-silicates: knots-and-crosses structures

Sodium chloride stannosilicate AV-13 (Na2.26SnSi3O9Cl0.26·xH2O) and zirconium and hafnium analogues of this material have been prepared and their structures solved from powder X-ray diffraction data using direct methods, and 23Na, 29Si and 119Sn solid-state NMR. AV-13 materials are small-pore solids, probably more adequately described as tunnel structures. The AV-13 framework consists of corner-sharing MO6 (M=Sn, Zr, Hf) octahedra and SiO4 tetrahedra. The latter form six-membered [Si6O18]12− rings, which are interconnected by MO6 octahedra. The structure is better understood by considering a three-dimensional knots-and-crosses lattice. In a given layer, successive distorted-cube M8 cages contain [Na6−x(H2O)x](H2O,Cl−) octahedra (knots) and cyclohexasilicate (crosses) units. While the former are extra-framework species, the six-membered rings are, of course, part of the framework. The cages are accessed via seven-membered [M3Si4O27]26− windows, with free aperture ca. 2.3×3.2 Å, one per each pseudo-cube face. Pilling up layers generates the structure, with knots-and-crosses alternating. The non-framework five-coordinated Na cations are disordered.