A molecular dynamics study of densification mechanisms in calcium silicate glasses CaSi2O5 and CaSiO3 at pressures of 5 and 10 GPa

Molecular dynamics is used to obtain models of (CaO)(x)(SiO2)(1-x) glasses, with compositions CaSi2O5 (x=0.33) and CaSiO3 (x=0.50), at pressures of 5 and 10 GPa. At 5 GPa there are increases in Ca and Si coordinations for x=0.33, whereas for x=0.50 there is distortion of CaON polyhedra but no substantial change in coordination. At 10 GPa the Ca coordination increases by similar to 20% for x=0.33 and by similar to 10% for x=0.50. This increase is due to increased Ca bonds to bridging oxygens (O-b), since nonbridging oxygens (O-nb) are already highly bonded to Ca, and the proportion of O-nb is decreasing due to changes in the silica network. At 10 GPa there are similar to 20% of fivefold and a few percent of sixfold coordinated Si. Since the new Si-O bonds involve the conversion of O-nb to O-b, there is a corresponding increase in the network connectivity. The x=0.50 glass is more resistant to deformation because there is less possibility to convert O-nb to O-b due to lower Si content. The changes in Ca-O, Si-Ca, and Ca-Ca correlations are predicted to produce changes in the x-ray diffraction structure factor S(Q), including a shift of the first sharp diffraction peak to higher Q values. (c) 2006 American Institute of Physics.