Molecular dynamics study of the thermal behaviour of silica glass/melt andcristobalite

The thermal behaviour of amorphous silica is compared with that of a high t emperature form of cristobalite using molecular dynamics (MD) simulations, in order to understand the thermal behaviour of silica glass from an atomis tic point of view. The MD simulations reproduce the alpha-beta transition o f cristobalite, the negative thermal expansion of beta -cristobalite, and t he density maximum of the silica melt. The thermal expansion of cristobalit e is closely related to the distances between the first neighbour silicon a toms, which depend on the average Si-O-Si angle. On the other hand, the the rmal expansion of amorphous silica is not directly correlated with the vari ation in the average Si-O-Si angle. The positive thermal expansion of silic a 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. Th e density anomaly of the silica melt is caused by two opposite factors in t he density variation with decreasing temperature: densification due to the increase in number of bridging bonds, and opening of the tetrahedral networ k in which 5- to 7-membered rings become dominant with a reduction in small er rings. (C) 2001 Elsevier Science B.V. All rights reserved.