Monte Carlo simulations of dielectric relaxation in Na-mordenites

The chemical composition of zeolites is defined by the average ratio between the number of 4-coordinated Si and Al atoms of the lattice (ratio Si/Al). Each Al atom adds a net negative charge to the otherwise SiO2 lattice which is counterbalanced by an extra-framework mobile cation. These counter-ions are responsible for the major polarization of the material at low frequencies (106 Hz). The nature, localization and diffusion of these cations depend on the Si/Al ratio and influence the catalytic properties of the material. Here, we present a joint theory-experiment study of these properties at the atomic level. The thermally stimulated depolarization current (TSDC) of Na-mordenites is measured for increasing Si/Al ratios. The analysis of these dielectric relaxation data leads to activation energy barriers for the Na+ “jumps” responsible for the polarization change. Using semi-empirical inter-atomic potentials and Monte Carlo algorithms we propose a possible mechanism for the cation motions occurring in TSDC experiments.