Dynamics of sodium in sodium disilicate: Channel relaxation and sodium diffusion

We use molecular-dynamics computer simulations to study the dynamics of amorphous (Na2O)(2)(SiO2). We find that the Na ions move in channels embedded in a SiO2 matrix. The characteristic distance between these channels gives rise to a prepeak in the structure factor at q approximate to 0.95 Angstrom(-1). The dynamics of sodium is given by a fast process which can be seen in the incoherent scattering function and a slow process which is seen in the coherent function. The relaxation time of the latter coincides with the alpha-relaxation time of the matrix. The Kohlrausch exponent of the fast process for q > 1.6 Angstrom(-1) is the same as the von Schweidler exponent for the slow one. Thus the two processes are closely related.