  We use inelastic neutron scattering and molecular dynamics simulation to
investigate the interplay between the structure and the fast sodium ion
diffusion in various sodium silicates. With increasing temperature and
decreasing density the structure factors exhibit an emerging prepeak around 0.9
A^-1. We show, that this prepeak has its origin in the formation of sodium rich
channels in the static structure. The channels serve as preferential ion
conducting pathways in the relative immobile Si-O matrix. On cooling below the
glass transition this intermediate range order is frozen in.
