On the effect of hydrostatic pressure on the auxetic character of NAT-type silicates

Materials with negative Poisson’s ratios (auxetics) exhibit the property of expanding laterally when uniaxially stretched and becoming narrower when compressed. A system which exhibits this unexpected property is natrolite (NAT), a zeolite which is auxetic in its (0 0 1) plane. Here, we examine the effect of external hydrostatic pressure on the crystal structure of hypothetical silicates equivalent to frameworks in the NAT group. We show that the crystal structure and mechanical properties of the SiO2 equivalents of the NAT, THO (thomsonite) and EDI (edingtonite) frameworks are highly pressure dependent and that these systems are most auxetic at non-ambient conditions, in particular at positive external hydrostatic pressures which are predicted to be approximately 2–8% of the bulk modulus. An attempt is made to explain this pressure dependency of the Poisson’s ratio in terms of the framework geometry and the deformation mechanism.