STRUCTURE AND DYNAMICS OF ALKALI SILICATE -GLASSES - GLASS-TRANSITION, RELAXATION, AND THE MIXED-ALKALI EFFECT

Molecular-dynamics simulation of lithium metasilicate (Li2SiO3) has be en performed. Saturation of the packing of the oxygen atoms is observe d near the glass transition temperature (T-g) by geometric analysis of the coordination polyhedra. Diffusion in the glassy state is found to be caused by jump motions of the atoms. The changes in dynamics near T-g are discussed in relation with the structural changes. A clear two -step relaxation was observed in a density correlation function (self- part) for Li ions following an exponential decay by vibrational motion . The origin of the stretched-exponential type decay (alpha-relaxation ) in a long time region is confirmed to be a waiting time distribution of the jump motions. The fractal dimension of space is also important to determine the relaxation behavior. The mechanism of a ''mixed alka li effect'' in LiKSiO3 has been also examined. The pathway of each alk ali metal ion in the mixture is found to be intercepted by each other. Cooperative jumps and back-correlation are found to play an important role in the ''mixed alkali effect''.