The adiabatic elastic constants ( C(ij)) 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, C44 exhibits a value extremely close to C11 rather than C12, 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.

We have presented the molecular-dynamics ͑MD͒ results for the temperature dependence of the adiabatic elastic constants C i j of ␣ and ␤ quartz, using a statistical fluctuation formula. It is noteworthy that the calculated C i j values are in a good agreement with the experimental values in the entire temperature range of 300-1100 K, including the ␣-␤ phase-transition region. We have confirmed that the net increase of bulk C i j 's in the ␤ phase can be attributed to the internal relaxations, which arise from the cooperative motions of corner-linked SiO 4 tetrahedra. Our MD simulations have revealed the existence of dynamical disorder in ␤ quartz at high temperatures, and its influence on the macroscopic elastic properties, in contrast to the ordered ␤-quartz structure model.

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