The local atomic environment of oxygen in silicate glasses from molecular dynamics

Recent molecular dynamics (MD) results for (Na2O)(x)(SiO2)(1-x) and (CaO)(x)(SiO2)(1-x) glasses show that co-ordination of bridging oxygen (O-b) by modifiers (M) is a normal structural feature. This can be explained as a consequence of the limitation on oxygen co-ordination in network oxides, a common rule of thumb being that total co-ordination of oxygen by (Si + M) is <= 4. This gives an upper limit on co-ordination of non-bridging oxygen (O-nb) by modifiers of N-onbM <= m with m = 3, corresponding to N-MOnb <= mv, where v is modifier valence. If modifier co-ordination exceeds this limit, i.e. N-MO > mv, then there is bonding Of Ob to modifiers, i.e. N-ObM > 0. This is the case in alkali and alkaline earth silicate crystals (apart from Be and Mg), and is predicted to be a feature of glasses in these systems. An illustration of the influence of oxygen co-ordination is given from MD models of (CaO)(0.33) (SiO2)(0.67) glass at pressures of 5 and 10 GPa. The main effect of densification is to increase the co-ordination of Ca by O-b. This can be understood because at 0 GPa the co-ordination Of O b by Ca is already high, with N-OnbCa similar to 2.7, but the co-ordination of O-b by Ca is less high, with N-ObCa similar to 1, and so can more easily increase.