Scaling of the structural relaxation in simulated liquid silica

A scaling law for the alpha relaxation time tau, involving the ratio of a density-dependent energy to the thermal energy, has been found to hold in many fragile glass-forming liquids. This scaling has been recently linked to a local quantity n(loc)(rho,T) relating the variation of tau with density to its variation with temperature. In many fragile liquids, the variation of n(loc)(rho,T) is weak enough to take it as constant over the experimental temperature and density domain. We show that simulated liquid silica has an opposite behavior; close to T-g, n(loc) is negative for moderate densities and exhibits a significant variation with rho and T, reaching positive values for higher temperature and/or densities. Moreover, those variations linearly correlate to a measure of the bonding properties of the liquid.