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 (Ob) 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 (Onb) by modifiers of NOnbM ⩽ m with m = 3, corresponding to NMOnb ⩽ mv, where v is modifier valence. If modifier co-ordination exceeds this limit, i.e. NMO > mv, then there is bonding of Ob to modifiers, i.e. NObM > 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 Ob. This can be understood because at 0 GPa the co-ordination of Onb by Ca is already high, with NOnbCa ∼ 2.7, but the co-ordination of Ob by Ca is less high, with NObCa ∼ 1, and so can more easily increase.