Molecular dynamics study of the mechanism of ion transport in lithium silicate glasses: Characteristics of the potential energy surface and structures

Characterization of the potential energy surface is one of the essential problems to understand the mechanism of the ion conduction in glasses. In this work, ion dynamics in several lithium silicate glasses are examined by using molecular dynamics simulations. The number of ion sites, site energy, and the respective structures were examined for both fast (diffusive) and slow (localized) ions. We have visualized ion sites using the molecular dynamics simulation data and obtained the number of sites without being affected by a cut off value. The number obtained for the Li2SiO3 glass is 8-10% larger than that of ions. The value is reasonable to explain the diffusion mechanism by cooperative jumps, since rapid decrease of transport property in cooperative dynamics is expected if the number of the sites is too small while the cooperative jumps may be scarcely observed when the number of the vacancy is too large. The percentage of the available sites for the Li4SiO4 in the glassy state was found to be almost the same as that for Li2SiO3, while the diffusion coefficient of Li in Li4SiO4 is larger than Li2SiO3. Increase of the diffusion coefficient with increasing alkali contents is easily explained by the contribution of the cooperative jumps but not by a simple mobile vacancy mechanism. Besides the characteristic sites for each slow (type A) and fast (type B) ions, many common sites for both type of ions are found, while the steepest descent energy distribution for these types of ions is quite similar. On the other hand, the partial pair correlation function, g(r) of A-A pairs is found to be quite different from that of B-B pairs. Therefore, microstructures related to the density fluctuation of the Li ions are important for the difference of slow and fast dynamics.