Static and dynamic properties of a viscous silica melt

We present the results of a large scale molecular dynamics computer simulat ion 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(10(2)) P. We show that even at temperatures as high a s 4000 K the structure of this system is very similar to the random tetrahe dral network found in silica at lower temperatures. The temperature depende nce 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 scatt ering data. At low temperatures the temperature dependence of the diffusion constants D shows an Arrhenius law with activation energies which are in v ery good agreement with the experimental values. With increasing temperatur e we find that this dependence shows a crossover to one which can be descri bed well by a power law, D-proportional to(T - T-c)(gamma). The critical te mperature T-c is 3330 K and the exponent gamma is close to 2.1. Since we fi nd a similar crossover in the viscosity, we have evidence that the relaxati on 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 obse rved in the product of the diffusion constant and the lifetime of a Si-O bo nd or the space and time dependence of the van Hove correlation functions. [S0163-1829(99)01329-6].