Molecular dynamics study of the thermal behaviour of silica glass/melt and cristobalite

The thermal behaviour of amorphous silica is compared with that of a high temperature form of cristobalite using molecular dynamics (MD) simulations, in order to understand the thermal behaviour of silica glass from an atomistic point of view. The MD simulations reproduce the α–β transition of cristobalite, the negative thermal expansion of β-cristobalite, and the density maximum of the silica melt. The thermal expansion of cristobalite is closely related to the distances between the first neighbour silicon atoms, which depend on the average Si-O–Si angle. On the other hand, the thermal expansion of amorphous silica is not directly correlated with the variation in the average Si–O–Si angle. The positive thermal expansion of silica glass is ascribed to the deformation of network-forming rings, not to the variations in the lengths of the nearest neighbour Si–O or Si–Si bonds. The density anomaly of the silica melt is caused by two opposite factors in the density variation with decreasing temperature: densification due to the increase in number of bridging bonds, and opening of the tetrahedral network in which 5- to 7-membered rings become dominant with a reduction in smaller rings.