Molecular dynamics analysis of negative Poisson ratios over the alpha-betatransition in cristobalite, SiO2

Cristobalite, a low-density polymorph of silicon dioxide (SiO2), shows an u nusual elastic behavior of negative Poisson ratio, contracting laterally wh en compressed and expanding laterally when stretched. We have successfully evaluated the adiabatic second-order elastic stiffness constants in the tem perature range of 300-1,800K, including a displacive phase transition regio n, with the equilibrium molecular dynamics method using the statistical flu ctuation formula. We have found that the cristobalite exhibits a negative P oisson ratio over this temperature range, however, with different mechanism s between low-temperature (alpha) and high-temperature (beta) phases.