Structural organisation in oxide glasses from molecular dynamics modelling

Classical molecular dynamics modelling has been used to obtain new models of 50CaO center dot 50P(2)O(5) and 50MgO center dot 50SiO(2) glasses and, together with previously published models of 63CaO center dot 37Al(2)O(3), and 50CaO center dot 50SiO(2) 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 center dot 3Al(2)O(3), 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 center dot 50SiO(2) glass show unusual behaviour because similar to 5% of oxygen is present as "non-network" oxygen, i.e. bonded only to Mg. The models show broader Q(n) 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 T-CaCa(r) which is found to be similar in the three glasses, and also similar to the previous experimental measurement for 50CaO center dot 50SiO(2) 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. (C) 2011 Elsevier B.V. All rights reserved.