Structure and Dynamics of Alkali Silicate Glasses. Glass Transition, Relaxation, and the Mixed Alkali Effect

Abstract

Molecular-dynamics simulation of lithium metasilicate (Li₂SiO₃) has been performed. Saturation of the packing of the oxygen atoms is observed 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 (α-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 alkali effect” in LiKSiO₃ has been also examined. The pathway of each alkali 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”.