ELEMENTARY DIFFUSIVE DISPLACEMENTS OF MODIFIER CATIONS IN NETWORK STRUCTURES

The molecular dynamic simulation technique was used to investigate cat ion mobilities in the network structures of alkali silicate liquid and glassy systems. The simulated diffusion trajectories were analyzed us ing a formalism devised for the description of noisy signals. The scal ing behavior of the power spectra computed from diffusion trajectories reveal correlation effects and non-Brownian patterns in the cation mo tion. On the timescale of observation, i.e., up to 2.4 x 10(-10) s, Br ownian diffusion could not be observed, a circumstance that requires, for the quantitative description of particle mobilities, that correlat ed displacements be taken into account. The model for the description of short time diffusion proposed here allows one to distinguish betwee n elementary jumps, frictional dissipation of phonon vibrational modes and slow structural relaxation. The influence of network rigidity, ca tion size and coordination, and thermal activation, on the mobility of the modifier cation and the correlation behavior in its motions is di scussed.