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

Recent molecular dynamics (MD) results for (Na ₂O) _x(SiO ₂) _1−x and (CaO) _x(SiO ₂) _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 O _b 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 (SiO ₂) _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 _nb by Ca is already high, with N _OnbCa ∼ 2.7, but the co-ordination of O _b by Ca is less high, with N _ObCa ∼ 1, and so can more easily increase.