An alternative explanation for the negative Poisson's ratios in α-cristobalite

Materials with a negative Poisson's ratio (auxetic) exhibit the unusual property of becoming wider when stretched and thinner when compressed. A naturally occurring auxetic material which has attracted a lot of research in recent years is α-cristobalite, a silicate for which negative Poisson's ratios have been experimentally measured. We present the results of force-field based molecular modelling studies which will provide an insight into nano-level deformations that occur when this silicate is subjected to externally applied uniaxial mechanical stresses. These results will suggest that the auxetic behaviour can be explained in terms of a ‘rotation of rigid units’ model and, more specifically we will show that the auxetic behaviour in the (0 1 0) and (1 0 0) planes may be explained in terms of ‘rotating rectangles’ which are the projections of the three-dimensional silicate framework in the (0 1 0) and (1 0 0) planes.