STRUCTURES OF SYNTHETIC K2MGSI5O12 LEUCITES BY INTEGRATED X-RAY-POWDER DIFFRACTION, ELECTRON-DIFFRACTION AND SI-29 MAS NMR METHODS

The structures of disordered and ordered varieties of the title compou nd have been determined using integrated TEM, MAS NMR and Rietveld ana lysis of synchrotron X-ray powder diffraction data. Both samples have a 'leucite-like' framework topology. The dry-synthesized sample is cub ic, Ia3d [a = 13.4190 (1) angstrom, V = 2416.33 (5) angstrom3] with di sordered Mg and Si in tetrahedral framework sites. The hydrothermally synthesized analogue is monoclinic, P2(1)/c [a = 13.168 (5), b = 13.65 2 (1), c = 13.072 (5) angstrom, beta = 91.69-degrees, V = 2348 (2) ang strom3], and has a fully ordered framework with four K, ten Si and two Mg sites per 24 O atoms (one quarter of the unit cell). Two of these Si sites are linked to Si tetrahedra only [Q4(4Si)], while the other e ight Si sites have one Mg and three Si tetrahedra as next-nearest neig hbours [Q4(3Si, 1 Mg)]. Q4(4Si) and Mg tetrahedra share opposite comer s of four rings. Si and Mg ordering is accompanied by a volume contrac tion of 2.8%. The X-ray structural data for the ordered sample allow t he Si-29 MAS NMR peaks to be assigned to particular Si tetrahedra, and thus to particular values of the mean T-O-T angle. The nature of the polymorphism between the disordered and ordered samples is discussed a nd related to the different synthesis conditions. Water in the hydroth ermal synthesis accelerates Si-Mg ordering, allowing the thermodynamic ally more stable phase to be formed.