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 (hexacelsian(LTA) and hexacelsian(FAU)) show that the phase transition near 58010 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.