Résumé
Chem. Geol. 174, 87-101 (2001) We present the results of large scale computer simulations in which we
investigate the static and dynamic properties of sodium disilicate and sodium
trisilicate melts. We study in detail the static properties of these systems,
namely the coordination numbers, the temperature dependence of the Q^(n)
species and the static structure factor, and compare them with experiments. We
show that the structure is described by a partially destroyed tetrahedral SiO_4
network and the homogeneously distributed sodium atoms which are surrounded on
average by 16 silicon and other sodium atoms as nearest neighbors. We compare
the diffusion of the ions in the sodium silicate systems with that in pure
silica and show that it is much slower in the latter. The sodium diffusion is
characterized by an activated hopping through the Si-O matrix which is frozen
with respect to the movement of the sodium atoms. We identify the elementary
diffusion steps for the sodium and the oxygen diffusion and find that in the
case of sodium they are related to the breaking of a Na-Na bond and in the case
of oxygen to that of a Si-O bond. From the self part of the van Hove
correlation function we recognize that at least two successive diffusion steps
of a sodium atom are spatially highly correlated with each other. With the same
quantity we show that at low temperatures also the oxygen diffusion is
characterized by activated hopping events.