Atomistic simulation of cristobalite at high temperature

Lattice-dynamics and molecular-dynamics simulations based on the Born model of solids have been used to investigate the structural disorder present in the beta phase of cristobalite. We used both shell and rigid-ion interatom ic potentials and compared with available experimental data the values pred icted for a large number of alpha -cristobalite properties at different tem peratures. Free-energy minimization at room temperature suggests the possib ility of three different but energetically similar cristobalite structures. Molecular-dynamics simulations point to dynamical disorder in beta cristob alite at high temperatures and show its relation to these three possible mo difications. This approach also demonstrates the influence of the alpha-bet a transition on structural disorder and assigns the unit-cell contraction o bserved at high temperature to the decrease of the first Si-Si distance due to the large thermal motion of oxygen atoms which cause the neighboring te trahedra to come closer Co each other.