High-temperature synchrotron powder diffraction investigation of thermal expansion, strain and microstructure for the co-elastic alpha <->beta hexacelsian transition

High-temperature investigations of thermal expansion, strain and microstruc ture of the alpha <--> beta-hexacelsian reversible phase transition are rep orted. Hexacelsian(LTA) and hexacelsian(FAU) were synthesised from LTA and FAU zeolites, respectively. Only the former show discontinuities near the t ransition temperature, similar to 580 K The discontinuity in the thermal ex pansion near the transition temperature for the alpha <--> beta-hexacelsian (LTA) suggest a co-elastic character for the phase transition. A distortion of the co-ordination polyhedra and ordering of framework cations induce th is phase transition. The absence of discontinuities for hexacelsian(FAU) su ggests non-existence of a phase transition. The similarities in the measure d and calculated values for beta-hexacelsian(LTA) and hexacelsian(FAU) sugg est that hexacelsian(FAU) crystallise exclusively as the beta-form.