Relaxation processes and the mixed alkali effect in alkali metasilicate glasses

A molecular dynamics simulation (MD) of lithium metasilicate (Li2SiO3) and related mixed alkali system (LiKSiO3) has been performed. Changes in the mean squared displacement and the corresponding clear two-step (beta and alpha 1) relaxations in a density correlation function have been observed at 700 K (self-part) for each ion in Li2SiO3 following an exponential decay by vibrational motion in a simulation up to 300 ps (run I). The mean squared displacement of the atoms shows the change in the slope at ca. 300 ps when the simulation is extended up to 1 ns (run II). Here we call the slowest relaxation (ca. 300 ps similar to) the alpha 2 region. Oscillation, which is clearer for O and Si than for Li, is found in the second (beta-relaxation) region of the function, which is attributed to the so called ''boson, peak'' Both tile beta-relaxation and the boson peak are found to be due to the correlated motion. The slower relaxation (alpha 1-relaxation) can be fitted to a stretched exponential form and the origin of this type of decay is confirmed to be waiting time distribution of jump motions. The back-correlated jumps also decrease the decay rate. Components A and B in alpha 1 and alpha 2 regions for Li ion are analyzed, where the Li ion of component A is located within the first neighboring sites and that of component, B moves longer than the nearest neighbor distances by cooperative jump motion. The component B shows accelerated dynamics larger than t-linear ones (similar to t(1.77)) in the region 50-300 ps, and the dynamics can be characterized as Levy flight. We have found that the contribution of the cooperative jumps decreases in the mixed alkali glass. This explains the maximum of the Haven ratio accompanied with the mixed alkali effect.