Hot-stage transmission electron microscopy study of phase transformations in hexacelsian (BaAl2Si2O8)
Phase transformations in synthetic hexacelsian were investigated by hot-stage transmission electron microscopy. A second phase transformation from an orthorhombic to hexagonal structure was identified in the synthetic hexacelsian at approximately 700 °C. The hexacelsian was found to exhibit a sequence of phase transformations on heating of hexagonal (P6 3 /mcm)-orthorhombic (Immm)-hexagonal (P6/mmm). Antiphase domain boundaries, which were observed in P6 3 /mcm and Immm phases, were absent in the P6/mmm phase. Crystal symmetries of the three phases were determined by convergent beam electron diffraction, and space group symmetries were derived by comparison of experimental selected area electron diffraction patterns with computer-simulated patterns.
The lithium- and sodium-enhanced transformation of Ba-exchanged zeolite LTA into celsian phase
Abstract The Li + - and Na + -doped hexacelsians (HC) synthesized from Ba-LTA synthetic zeolite as precursors were used for preparation of monoclinic celsians (MC). The doped pure MC and mixture HC/MC species were obtained by thermally induced polymorphous transformation at 1200 °C. Synthesized-doped MC have been characterized by X-ray powder diffraction and spectroscopic (infrared, Raman and 29 Si magic angle spinning nuclear magnetic resonance) methods. The obtained results suggest that in all investigated samples Na + or Li + dopants were incorporated in MC crystal structures during thermal transformation of a zeolite. It has been shown that HC→MC transformation depends on molar fractions of alkali cations. These findings combined with used temperature/time conditions could help in optimization of MC synthesis route.
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