High-temperature synchrotron powder diffraction investigation of thermal expansion, strain and microstructure for the co-elastic α ↔ β hexacelsian transition

Abstract

High-temperature investigations of thermal expansion, strain and microstructure of the α ↔ β-hexacelsian reversible phase transition are reported. HexacelsianLTA and hexacelsianFAU were synthesised from LTA and FAU zeolites, respectively. Only the former show discontinuities near the transition temperature, approx. 580 K. The discontinuity in the thermal expansion near the transition temperature for the α ↔ β-hexacelsianLTA suggest a co-elastic character for the phase transition. A distortion of the co-ordination polyhedra and ordering of framework cations induce this phase transition. The absence of discontinuities for hexacelsianFAU suggests non-existence of a phase transition. The similarities in the measured and calculated values for β-hexacelsianLTA and hexacelsianFAU suggest that hexacelsianFAU crystallise exclusively as the β-form.