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 alpha-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 (010) and (100) planes may be explained in terms of 'rotating rectangles' which are the projections of the three-dimensional silicate framework in the (010) and (100) planes. (c) 2006 Published by Elsevier B.V.