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Persistent URL http://purl.org/net/epubs/work/34545
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Record Id 34545
Title Structures of Synthetic K2MgSi5 O12 Leucites by Integrated X-ray Powder Diffraction, Electron Diffraction and 29Si MAS NMR Methods
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Abstract 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.
Organisation SRS , 2.3 , SERC
Keywords Materials , Phase Transitions , Crystal Structure , Behavior , Magnesium
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Language English (EN)
Type Details URI(s) Local file(s) Year
Journal Article Acta Crystallogr B 50 (1994): 31-41. doi:10.1107/S0108768193008754 li0152.pdf 1994