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Structure, diffusion and orientational freezing in lithium metasilicate

We report on the dynamic and structural characterization of lithium metasilicate $Li_2SiO_3$, a network forming ionic glass, by means of molecular dynamics simulations. The system is characterized by a network of $SiO_4$ tetrahedra disrupted by $Li$ ions which diffuse through the network. Measures of mean square displacement of $Si$ and $O$ atoms allow us to identify a temperature at which tetrahedra stop moving relative to each other. This temperature $T_c\approx 1500\,K$ can be characterized within the framework of mode coupling theory. At a much lower temperature $T_g\approx 1000\,K$, a change in the slope of the volume versus temperature data allows to single out the glass transition. We find signatures of both transitions in structural order parameters, related to the orientation of tetrahedra. Going down in temperature we find that, around the mode coupling transition temperature, a set of order parameters which measure the relative orientation of tetrahedra cease to increase and stay constant below $T_c$. Another well known measure of orientational order, the bond orientational order parameter, which in the studied system measures local order within single tetrahedrons, is found to continue growing below $T_c$ until $T_g$, below which it remains constant. Our results allow to relate two characteristic dynamic transitions with corresponding structural transitions, as observed in two different orientational order parameters. Furthermore, the results indicate that the network of thetrahedra continue to relax well below the point where neighboring tetrahedra cannot rearrange relative to each other, and the glass is reached only upon a process of relaxation of atoms which form the thetrahedron, as quantified by the change in the bond orientational order parameters.