Structural signatures of (two) characteristic dynamical temperatures in lithium metasilicate

We report on the dynamic and structural characterization of lithium metasilicate Li2SiO3, a network-forming ionic glass, by means of molecular dynamics simulations. The system is characterized by a network of SiO4 tetrahedra disrupted by Li ions which diffuse through the network. Measures of mean square displacement and the diffusion constant of Si and O atoms allow us to identify the mode-coupling temperature, Tc ≈ 1500 K. At a much lower temperature, a change in the slope of the specific volume versus temperature singles out the glass transition at Tg ≈ 1000 K, the temperature below which the system goes out of equilibrium. We find signatures of both dynamical temperatures in structural order parameters related to the orientation of the tetrahedra. At lower temperatures we find that a set of order parameters which measure the relative orientation of neighbouring tetrahedra cease to increase and stay constant below Tc. Nevertheless, the bond orientational order parameter, which in this system measures local tetrahedral order, is found to continue growing below Tc until Tg, below which it remains constant. Although these structural signatures of the two dynamical temperatures do not imply any real thermodynamic transition in terms of the order parameters, they do give insight into the relaxation processes that occur between Tc and Tg, in particular they allow us to characterize the nature of the crossover happening around Tc.