A molecular dynamics study of the atomic structure of (CaO)x(SiO2)1-x glasses

The local atomic environment of Ca in ( CaO)(x)( SiO2)(1-x) glasses is of interest because of the role of Ca in soda-lime glass, the application of calcium silicate glasses as biomaterials, and the previous experimental measurement of the Ca-Ca correlation in CaSiO3 glass. Molecular dynamics has been used to obtain models of (CaO) x( SiO2)(1-x) glasses with x) 0, 0.1, 0.2, 0.3, 0.4, and 0.5, and with similar to 1000 atoms and size similar to 25 angstrom. As expected, the models contain a tetrahedral silica network, the connectivity of which decreases as x increases. In the glass-forming region, i. e., x = 0.4 and 0.5, Ca has a mixture of 6- and 7- fold coordination. Bridging oxygen makes an important contribution to the coordination of Ca, with most bridging oxygens coordinated to 2 Si plus 1 Ca. The x = 0.5 model is in reasonable agreement with previous experimental studies, and does not substantiate the previous theory of cation ordering, which predicted Ca arranged in sheets. In the phaseseparated region, i. e., x = 0.1 and 0.2, there is marked clustering of Ca.