Crystal structure, variation of cell parameters, and peak broadening of levyne (NaCa2.5[Al6Si12O36].18H2O), synthesized from methylquinuclidine-containing systems at different R = SiO2/Al2O3 ratio (10 < R < 35), were studied using synchrotron and conventional X-ray powder diffraction. Rietveld structure refinements of three samples (R = 35, 20, 10) showed that, with increasing Na content, only one of the two statistical orientations of the occluded methylquinuclidine ion (MeQ+) was preserved. The variation of the unit-cell parameters as a function of the Si/(Si + Al) ratio in synthetic levynes compared with that in natural Ca-rich and Na-exchanged samples suggests that decreasing the Si/(Al + Si) ratio caused an enlargement of the lateral dimensions and a contraction of the vertical dimensions, which were counter-balanced by the presence of extraframework cations within the levyne cages. From an analysis of peak broadening, the particle size of synthetic levynes decreased with the increasing Al content in the precursor gel. The compositional fields of natural (R = 2.0 - 2.5) and synthetic (R < 10) samples suggest that, for a given tetrahedral composition, levyne crystallization is favored by the availability of specific cations (e.g., Ca for low SiO2/Al2O3 ratios), the incorporation of which in the levyne cages reflected geometric constraints on lattice parameters.
Conventional and synchrotron X-ray powder diffraction study of synthetic levynes: comparison with natural levynes.
CRUCIANI, Giuseppe;
2000
Abstract
Crystal structure, variation of cell parameters, and peak broadening of levyne (NaCa2.5[Al6Si12O36].18H2O), synthesized from methylquinuclidine-containing systems at different R = SiO2/Al2O3 ratio (10 < R < 35), were studied using synchrotron and conventional X-ray powder diffraction. Rietveld structure refinements of three samples (R = 35, 20, 10) showed that, with increasing Na content, only one of the two statistical orientations of the occluded methylquinuclidine ion (MeQ+) was preserved. The variation of the unit-cell parameters as a function of the Si/(Si + Al) ratio in synthetic levynes compared with that in natural Ca-rich and Na-exchanged samples suggests that decreasing the Si/(Al + Si) ratio caused an enlargement of the lateral dimensions and a contraction of the vertical dimensions, which were counter-balanced by the presence of extraframework cations within the levyne cages. From an analysis of peak broadening, the particle size of synthetic levynes decreased with the increasing Al content in the precursor gel. The compositional fields of natural (R = 2.0 - 2.5) and synthetic (R < 10) samples suggest that, for a given tetrahedral composition, levyne crystallization is favored by the availability of specific cations (e.g., Ca for low SiO2/Al2O3 ratios), the incorporation of which in the levyne cages reflected geometric constraints on lattice parameters.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
