Mechanism for negative Poisson ratios over the α-β transition of cristobalite, SiO2:: A molecular-dynamics study

The adiabatic elastic constants (Cii) of cristobalite have been evaluated successfully over the temperature range of 300-1800 K using the molecular-dynamics method with a Fluctuation formula. Cristobalite shows a negative Poisson ratio over this temperature range. However, the mechanisms differ between the alpha and beta phases. In the cubic beta phase, C-44 exhibits a value extremely close to C-11 rather than C-12. in contrast to the Cauchy relation. This predicts a remarkable property that the longitudinal and transverse velocities coincide for the acoustic waves propagating along the [100] direction.