Molecular dynamics study of the influence of mobile cations on the reconstruction of an irradiated silicate glass

The effect of ballistic collisions in a simplified nuclear glass was investigated by molecular dynamics. Systematic results were obtained in the 0–16 keV energy range. Following a damage peak, reconstruction of the glass structure was observed in terms of the overall degree of polymerization. The reconstruction was facilitated by the presence of mobile cations. The dynamics of restoration of the SiO4 tetrahedrons during displacement cascades and during the formation of a structure from a random configuration can be fitted to curves corresponding to the same analytic formula. This similarity allowed us to examine the influence of mobile cations (alkalis or alkaline earths) on the formation of SiO4 entities in smaller systems formed from random configurations. The formation rate of SiO4 tetrahedrons accelerates with the Na2O or CaO percentage to reach a maximum rate above about 10 mol% Na2O or CaO. This threshold corresponds to the disappearance in the glass structure of zones comprising a central bridging oxygen surrounded by two first-neighbor bridging oxygen rings. No mixed alkali effect was observed in systems containing both Na and Cs because the formation of SiO4 does not require long-range diffusion of mobile cations.