Molecular dynamics simulations of vitreous silica structures

Molecular dynamics simulations have been used to investigate the structural changes in cristobalite and silica glass at high temperatures. We used potentials which have weaker electrostatic interactions than the more traditionally used TTAM or KFBS potentials. Our simulations reproduce the melting temperature of cristobalite and the glass transition temperature of silica glass to within 20%, and are more accurate than previous reported studies. The calculated results also reproduce the density maxima around 1800 K for cristobalite and 1700 K for silica glass. The detailed analysis of the predicted structural changes suggests that both density anomalies originate from a common feature. In either case, when the temperature is increased up to the local density maximum, the pronounced densification occurs because of bond-weakening, regardless of rearrangement of Si-O rings. In the reverse direction, when the temperature is decreased from the local density maximum, locally favored structuring due to the stabilization of bond angles and torsional angles is dominant and the density decreases. Two factors, dense packing and locally favored structuring, compete with each other in the region of density anomaly. (C) 2004 Elsevier B.V. All rights reserved.