Lattice Dynamics Simulation of Thermal Contraction of Faujasites

Negative coefficients of thermal expansion have been reported for certain zeolite structures, including LTA, NaX, and DAY, with immediate consequences on the mechanical stability and performance of the supported membranes in gas separations. This unusual behavior is typically attributed to the systematic rotation of the framework tetrahedrals that operate as rigid units. A new interatomic potential is proposed in this work and used in lattice dynamics calculations for the computation of the thermal expansion coefficient as well as of the elastic and bulk moduli of zeolite faujasite with different aluminum contents. Comparison of the present simulations with literature measurements as well as with experimental data carried out in this work showed that the dealuminated faujasite (DAY) crystals contract upon heating over the entire temperature range examined, whereas aluminosilicate NaX contracts up to the ambient temperature and then expands with increasing temperature. Our analysis showed that the effect of temperature on the faujasite unit cell volume depends on the rigidity of the SiO4 and AlO4 tetrahedra as well as on their ability to rotate around their corner sharing oxygen atoms.