Molecular dynamics simulation of ion dynamics in glassy ionic conductors: Evidence of the primitive ion hopping process

Experiments have shown that the properties of the dynamics of ions in ionically conducting glasses are similar to the dynamics of molecules in glass-forming substances. The short time dynamics of ions when caged are manifested as the measured nearly constant loss at high frequencies seen also in glass-forming substances. The conductivity relaxation of ions at long times has properties similar to the terminal structural alpha-relaxation of glass-formers. However, while there is present the ubiquitous Johari-Goldstein beta-relaxation playing the important role as the precursor of the alpha-relaxation, such corresponding relaxation has not been detected by experiment. We report molecular dynamics simulation of Li ion dynamics in lithium metasilicate glass performed specifically to look for this secondary relaxation. The data presented indicate its presence. Further support comes from conductivity relaxation data of ionic liquids. In ionic liquids, ions are molecules that have permanent dipole moments. Combined translational and rotational motion of the molecular ions should enable the primitive relaxation to be observed as a secondary relaxation in the dielectric loss spectra. From published data we calculate the primitive relaxation time, tau(0), of the Coupling Model. It is nearly the same as the relaxation time, tau(beta), of the observed secondary relaxation. (c) 2006 Elsevier B.V. All rights reserved.