Monoclinic-Orthorhombic Phase Transition in ZSM-5 Zeolite: Spontaneous Strain Variation and Thermodynamic Properties

Modeling the ferroelastic properties of the monoclinicorthorhombic phase transition of ZSM-5 zeolites is very relevant to the understanding of the effects of lattice strain on adsorption and diffusion properties of these microporous materials widely used in catalysis and water treatment. Using very accurate synchrotron X-ray diffraction data, we report here the analysis of spontaneous strain variation across the transition. According to the Landau theory, the behavior of the order parameter reveals the tricritical character of this transition from ferroelastic to paraelastic phase in ZSM-5. Combination of results from Landau analysis with calorimetric data allows the calculation of thermodynamic quantities which are in good agreement with the experimentally derived ones.