Compressibility, kinetics, and phase transition in pressurized amorphous silica

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Abstract
We model the process of densification of silica glass using molecular dynamics simulation in order to resolve the current controversy regarding the existence of the first-order phase transition in this material. We propose the picture in which the structural changes start to take place in the pressure window between 3 and 5 GPa, after which significant modifications take place with the structural breakdown in the medium range. We also study microscopic processes behind temperature-induced volume decrease of pressurized glass, seen experimentally. We simulate this process and observe similar negative thermal swelling, accompanied by considerable rebonding and relaxations processes. Global nature of rebonding, resulting from the extended character of floppy modes present in silica glass, yields a large value of temperature-induced densification. The densified structure shows broadening of the rings distribution, and we identify the microscopical changes that lead to the breakdown of the medium-range structure. The interesting observation from the long annealing of pressuried glass is the large-amplitude cooperative flow of atoms, which takes place as the structure relaxes through continuous rebonding and relaxation events.