Evidence for Ionic Diffusion in Dynamic Light Scattering from Glass-forming Sodium Borate Melts

Abstract

Photon correlation spectroscopy conducted on glass-forming sodium borate melts provides a measure of the dynamic structure factor of these network-forming liquids over a broad time window and large temperature range. The dynamics are complex and include a non-Arrhenius temperature dependence and a non-exponential response to perturbations both of which are observed in the present measurements and both of which evolve with changes in the density of covalent bonds. An unexpected feature is the occurrence of a second, much slower relaxation process that develops only in the sodium-modified borates with up to 10 mol% sodium oxide. This second process appears to be caused by the diffusion of sodium ions as they undergo random ion hopping between charge-compensating sites in the network on a timescale equal to that of the viscous relaxation.