A molecular dynamics study 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 ∼1000 atoms and size ∼25 Å. 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 phase-separated region, i.e., x = 0.1 and 0.2, there is marked clustering of Ca.