Phase transition in hexacelsian at about 580 K

Abstract

Temperature-induced structure and microstructure changes in hexacelsians (BaAl₂Si₂O₈) that have been synthesised from the Ba-exchanged LTA and FAU zeolites (hexacelsian_LTA and hexacelsian_FAU) show that the phase transition near 580 K exists only in hexacelsian_LTA. 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.

Keywords:

• Hexacelsian
• Phase transition
• Ferroelastic
• Landau theory