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 interatomic potentials and compared with available experimental data the values predicted for a large number of alpha -cristobalite properties at different temperatures. Free-energy minimization at room temperature suggests the possibility of three different but energetically similar cristobalite structures. Molecular-dynamics simulations point to dynamical disorder in beta cristobalite at high temperatures and show its relation to these three possible modifications. This approach also demonstrates the influence of the alpha-beta transition on structural disorder and assigns the unit-cell contraction observed 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 tetrahedra to come closer Co each other.