Molecular dynamics study of single and mixed alkali metasilicates-spatial and temporal characterization of the dynamics in the supercooled liquid and glassy states

A molecular dynamics simulation for single and mixed alkali metasilicates has been performed. The motion of lithium ions in Li2SiO3 can be divided into slow (type A) and fast (type B) categories clearly in the glassy state. It turns out that the slow dynamics are caused by the localized jump motions with a significantly long tail of the waiting time distribution. On the other hand, fast dynamics of the lithium ions are caused by successive co-operative jump motions and is identified as Levy flight. Furthermore, time intervals of the jump events were significantly shortened due to cooperative jumps for the fast dynamics in a short time region. The temporal and/or spatial origins determine the characteristics of both dynamics. A mixed alkali effect in LiKSiO3 occurs essentially due to mutual interception of jump paths of both types of mobile ions. In other words, this effect is clearly due to a spatial origin. It turns out that the fast component is nearly absent in the mixed alkali system, because the interception of the path suppresses the co-operative forward-correlated jump process. Some characteristics of the dynamics in super-cooled liquid states will also be discussed. (C) 2002 Elsevier Science B.V. All rights reserved.