Monte Carlo simulations of dielectric relaxation in Na-mordenites

The chemical composition of zeolites is defined by the average ratio betwee n 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 whic h is counterbalanced by an extra-framework mobile cation. These counter-ion s are responsible for the major polarization of the material at low frequen cies (10(6) Hz). The nature, localization and diffusion of these cations de pend on the Si/Al ratio and influence the catalytic properties of the mater ial. Here, we present a joint theory-experiment study of these properties a t the atomic level. The thermally stimulated depolarization current (TSDC) of Na-mordenites is measured for increasing Si/Al ratios. The analysis of t hese 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 me chanism for the cation motions occurring in TSDC experiments. (C) 2001 Else vier Science B.V. All rights reserved.