STRUCTURES OF SYNTHETIC K2MGSI5O12 LEUCITES BY INTEGRATED X-RAY-POWDER DIFFRACTION, ELECTRON-DIFFRACTION AND SI-29 MAS NMR METHODS
UNSPECIFIED. (1994) STRUCTURES OF SYNTHETIC K2MGSI5O12 LEUCITES BY INTEGRATED X-RAY-POWDER DIFFRACTION, ELECTRON-DIFFRACTION AND SI-29 MAS NMR METHODS. ACTA CRYSTALLOGRAPHICA SECTION B-STRUCTURAL SCIENCE, 50 (Part 1). pp. 31-41. ISSN 0108-7681Full text not available from this repository.
The structures of disordered and ordered varieties of the title compound have been determined using integrated TEM, MAS NMR and Rietveld analysis of synchrotron X-ray powder diffraction data. Both samples have a 'leucite-like' framework topology. The dry-synthesized sample is cubic, Ia3d [a = 13.4190 (1) angstrom, V = 2416.33 (5) angstrom3] with disordered Mg and Si in tetrahedral framework sites. The hydrothermally synthesized analogue is monoclinic, P2(1)/c [a = 13.168 (5), b = 13.652 (1), c = 13.072 (5) angstrom, beta = 91.69-degrees, V = 2348 (2) angstrom3], 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 eight Si sites have one Mg and three Si tetrahedra as next-nearest neighbours [Q4(3Si, 1 Mg)]. Q4(4Si) and Mg tetrahedra share opposite comers of four rings. Si and Mg ordering is accompanied by a volume contraction of 2.8%. The X-ray structural data for the ordered sample allow the 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 and related to the different synthesis conditions. Water in the hydrothermal synthesis accelerates Si-Mg ordering, allowing the thermodynamically more stable phase to be formed.
|Item Type:||Journal Article|
|Subjects:||Q Science > QD Chemistry|
|Journal or Publication Title:||ACTA CRYSTALLOGRAPHICA SECTION B-STRUCTURAL SCIENCE|
|Publisher:||MUNKSGAARD INT PUBL LTD|
|Official Date:||1 February 1994|
|Number of Pages:||11|
|Page Range:||pp. 31-41|
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