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.
Phase transition in hexacelsian at about 580 K
Temperature-induced structure and microstructure changes in hexacelsians (BaAl2Si2O8) that have been synthesised from the Ba-exchanged LTA and FAU zeolites (hexacelsianLTA and hexacelsianFAU) show that the phase transition near 580 K exists only in hexacelsianLTA. The X-ray powder diffraction method has been used to follow the evolution of the structure during the phase transition, as described here. The excess thermodynamic quantities Gibbs free energy (G), entropy (S) and enthalpy (H) are obtained through the Landau theory of phase transition. The constants of proportionality between the G and ordering parameter (Q) are: h = −170345 J mol−1, a = −66.6 J mol−1 K−1 and b = −410534 J mol−1. The abrupt change in the trigonal distortion of the single six-member tetrahedral [SiO4]4− and [AlO4]5− ring near 580 K is responsible for the phase transition. The phase transition is non-convergent, ferroelastic, pure and proper.
NEUTRON-DIFFRACTION STUDY OF THE STRUCTURES OF FERROELECTRIC AND PARAELECTRIC AMMONIUM SULFATE
Three‐dimensional neutron‐diffraction data have been collected for both the room‐temperature (paraelectric) and the low‐temperature (ferroelectric) phases of ammonium sulfate. Both phases are ordered. The transition, occurring at 223°K and involving a change from Space Group Pnam at room temperature to Space Group Pna21 in the ferroelectric phase, results in somewhat stronger hydrogen bonds in the ferroelectric phase, although there is no dramatic change in the barrier to rotation of the ammonium ions as a result of the transition. In both phases the two crystallographically independent ammonium ions have quite similar rotational barriers and hydrogen‐bond strengths. The transition also results in less distorted ammonium ions in the ferroelectric phase. At room temperature, there are only two hydrogen bonds with O···H distances less than 2.0 A. In the ferroelectric phase there are six. The mean distance is decreased by 0.1 A in the ferroelectric phase, although the range of distances remains the same. The...