Anomalous enthalpy relaxation in vitreous silica

It is a challenge to calorimetrically determine the glass transition temperature (T-g) of vitreous silica. Here, we demonstrate that this challenge mainly arises from the extremely high sensitivity of the T-g to the hydroxyl content, but also from decrease of the T-g with repeating the calorimetric scans. It is known that the liquid fragility (i.e., the speed of the viscous slow-down of a supercooled liquid at its T-g during cooling) has impact on enthalpy relaxation in glass. Here, we find that vitreous silica (as a strong system) exhibits striking anomalies in both glass transition and enthalpy relaxation compared to fragile oxide systems. The anomalous enthalpy relaxation of vitreous silica is discovered by performing the hyperquenching-annealing-calorimetry experiments. We argue that the strong systems like vitreous silica and vitreous Germania relax in a structurally cooperative manner, whereas the fragile ones do in a structurally independent fashion. We discuss the origin of the anomalous enthalpy relaxation in the HQ vitreous silica.