On the auxetic properties of solid benzene

Abstract

Auxetic materials exhibit the unusual property of expanding when stretched and getting thinner when compressed, i.e. they exhibit negative Poisson's ratios. Recent, ab-initio based studies carried out by Kearley et al. suggest that solid benzene may exhibit on-axis auxeticity. Here, we performed further simulations in an attempt to reproduce this claimed auxeticity using force-filed based simulations. We found that most of force-fields used could successfully reproduce the crystal structure of solid benzene but none of these force-fields could reproduce the predicted on-axis auxeticity by Kearley et al. We also performed additional simulations and analysis which suggested that the mechanical properties of solid benzene are highly strain dependant but no evidence of on-axis auxeticity was found. This is very significant as it suggests that either the force-fields that we have been using are not suitable to model non-bond interactions in solid benzene or that the results of Kearley et al. were solely an artefact of the simulation method they used. However, when we performed an off-axis analysis we found a window of auxeticity in the (010) and (100) planes with a maximum for loading at ~45° to either the [100] or the [001] direction and 35° to the [010] direction respectively. This auxeticity can be explained in terms of slight reorientations of the benzene rings within the crystal.