On the atomic level deformations in the auxetic zeolite natrolite

Auxetics (i.e. systems with negative Poisson's ratios) exhibit the unexpected feature of becoming fatter when stretched and narrower when compressed. A particular class of materials which has attracted significant attention in recent years in view of its potential for auxeticity is that of zeolites, in particular, the zeolite NAT where auxeticity has now been experimentally measured. Here we use molecular modelling to accurately reproduce through force-field based simulations the experimentally measured negative Poisson's ratios in NAT. We also simulate and measure the atomic level deformations that result when NAT is mechanically deformed and use these to evaluate whether existing models can be used to explain the negative Poisson's ratios in natrolite. In particular we show that existing 2D models involving 'rotating semirigid quadrilaterals' can explain some aspects of the behaviour of NAT and make, suggestions how such models can be improved so as to provide a better description of NAT in the auxetic (001) plane.