Molecular dynamics studies of the vitreous state: Simple ionic systems and silica

Abstract

Some applications of molecular dynamics calculations to the vitreous state have been examined for simple ionic MX and MX2 type glasses. The MX system which corresponds to a hypothetical vitreous and supercooled amorphous KCl is found to undergo an isobaric transition in the region of 0.3 Tf and an isothermal transition at around 35 kbar (T=Tf). The transition is evidenced by a discontinuity in second order thermodynamic properties and may be associated with a virtual disappearance of translational diffusion. By employing a variety of irreversible stress histories, the vitreous state in this case is shown to be insensitive to the thermodynamic stress history up to a maximum computationally permissible relaxation time of around 10−10 sec. Simulations are also reported for some ionic liquids of MX2 stoichiometry which, in contrast with KCl, have glass-forming ability (BeF2, ZnCl2, and SiO2). Although the relaxation times involved in laboratory glass formation far exceed the long-time limit of computer ’’experiment,’’ the configurationally arrested states which are produced by simulation exhibit many characteristic features of the experimental glasses. Changes of heat capacity consequent on configurational arrest in the case of KCl are similar in magnitude to those observed at the experimental glass transition of simple ionic glasses. The radial distribution for simulated SiO2 ’’glass’’ is very similar to that for vitreous silica, and a simulated compression–vitrification–expansion cycle for this system produces a ’’densified’’ glass, as observed in laboratory experiments. A rattle-and-correlated-jump mechanism appears suitable for the description of diffusion in this liquid. Limitations and further applications of MD calculations pertaining to the vitreous state are discussed.