Structural organisation in oxide glasses from molecular dynamics modelling

Classical molecular dynamics modelling has been used to obtain new models of 50CaO·50P2O5 and 50MgO·50SiO2 glasses and, together with previously published models of 63CaO·37Al2O3, and 50CaO·50SiO2 glasses, these have been inspected to evaluate structural features. For the first time, models of glasses near the eutectic in three systems, aluminate, silicate, and phosphate, with the same modifier, Ca, have been compared. All have short range order which is similar to that in crystals of the same composition, 5CaO·3Al2O3, CaSiO3 and Ca(PO3)2. There is a clear trend in bonding of bridging oxygen to Ca, which is dominant in aluminate glass, common in silicate glass, and absent in phosphate glass. Preliminary results for 50MgO·50SiO2 glass show unusual behaviour because ~ 5% of oxygen is present as “non-network” oxygen, i.e. bonded only to Mg. The models show broader Qn distributions than seen in NMR experiments, and this remains an area for improvement of MD modelling of glasses. The distributions of Ca in the models have been studied using the pair distribution function TCaCa(r) which is found to be similar in the three glasses, and also similar to the previous experimental measurement for 50CaO·50SiO2 glass. The distributions of Ca are markedly different in the glasses compared to the crystals, being isotropic in the former and anisotropic in the latter, which should be a factor in glass forming ability.

Research Highlights► First comparison of the structure of Ca aluminate, silicate and phosphate glasses. ► Bonding of Ca to bridging oxgyens follows trend across glass systems. ► Clear evidence for non-network oxygen in Mg silicate glass. ► Distribution of Ca ions is similar in each glass, and is different to crystals.