Static and dynamic properties of a viscous silica melt

We present the results of a large scale molecular dynamics computer simulation in which we investigated the static and dynamic properties of a silica melt in the temperature range in which the viscosity of the system changes from O(10^-2) P to O(10²) P. We show that even at temperatures as high as 4000 K the structure of this system is very similar to the random tetrahedral network found in silica at lower temperatures. The temperature dependence of the concentration of the defects in this network shows an Arrhenius law. From the partial structure factors we calculate the neutron scattering function and find that it agrees very well with experimental neutron scattering data. At low temperatures the temperature dependence of the diffusion constants D shows an Arrhenius law with activation energies which are in very good agreement with the experimental values. With increasing temperature we find that this dependence shows a crossover to one which can be described well by a power law, D~(T-T_c)^γ. The critical temperature T_c is 3330 K and the exponent γ is close to 2.1. Since we find a similar crossover in the viscosity, we have evidence that the relaxation dynamics of the system changes from a flowlike motion of the particles, as described by the ideal version of mode-coupling theory, to a hoppinglike motion. We show that such a change of the transport mechanism is also observed in the product of the diffusion constant and the lifetime of a Si-O bond or the space and time dependence of the van Hove correlation functions.