We present the results of large scale computer simulations in which weinvestigate the static and dynamic properties of sodium disilicate and sodiumtrisilicate 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. Weshow that the structure is described by a partially destroyed tetrahedral SiO_4network and the homogeneously distributed sodium atoms which are surrounded onaverage by 16 silicon and other sodium atoms as nearest neighbors. We comparethe diffusion of the ions in the sodium silicate systems with that in puresilica and show that it is much slower in the latter. The sodium diffusion ischaracterized by an activated hopping through the Si-O matrix which is frozenwith respect to the movement of the sodium atoms. We identify the elementarydiffusion steps for the sodium and the oxygen diffusion and find that in thecase of sodium they are related to the breaking of a Na-Na bond and in the caseof oxygen to that of a Si-O bond. From the self part of the van Hovecorrelation function we recognize that at least two successive diffusion stepsof a sodium atom are spatially highly correlated with each other. With the samequantity we show that at low temperatures also the oxygen diffusion ischaracterized by activated hopping events.
展开▼
