On the behaviour of natrolite under hydrostatic pressure

Natrolite (NAT) is a zeolite which exhibits negative Poisson's ratios (auxetic) in its (001) plane (i.e. expands laterally when uniaxially stretched and becomes narrower when compressed), a property which has now been verified experimentally. It was recently shown that the empty SiO2 equivalent of the NAT framework is more auxetic if it is tested in conditions of positive external hydrostatic pressures. Here, we report results of additional simulations which show that the extent of auxetic behaviour of NAT having the standard composition (i.e. Na2(Al2Si3O10).2H2O) is also highly dependent on the magnitude of external pressure where it is tested. However, in contrast with the behaviour of its empty silica equivalent, NAT exhibits maximum auxetic behaviour at negative external hydrostatic pressures (i.e. partial vacuum conditions). This is very significant as it highlights the fact that auxetic behaviour may be more prominent in materials if they are tested at non-ambient pressure conditions. An attempt is made to explain this behaviour.