High-pressure behaviour of yugawaralite at different water content: an ab initio study.

The influence of the water content in a partially occupied water site in yugawaralite on the pressure-induced structural modifications of this zeolite has been studied by means of ab initio molecular dynamics simulations. The effect of the water site occupancy has been singled out by comparing results from simulations, performed with the cell parameters determined via XRPD at rP and 8.8 GPa, on two model systems whose cell stoichiometry differs only by the presence of a water molecule in such a site. Our results show that the global P-induced changes of the framework structure upon compression are mainly attributable to the template effect of Ca extra-framework cations, while the presence or absence of water in a partially occupied site affects the framework structure only at short range distances from the site. However, the overall water content plays a fundamental role in stabilizing/destabilizing the zeolite structure as a function of pressure.