Molecular dynamics study of temperature dependence of volume of amorphous silica

Vitreous silica (v-SiO2) exhibits an unusual volume–temperature dependence; the volume has a maximum and minimum at certain temperatures, T* and Tp respectively. The origin of such an unusual thermal property was studied by molecular dynamics computer simulation. Four types of potentials were compared: pairwise additive potentials using exact the Coulomb term (E2) and screened Coulomb term (S2), and two potentials including a three-body term for reproducing the SiO4 tetrahedral structure, one using the exact Coulomb term (E3), and the other one a screened Coulomb term (S3). The volume minimum could be reproduced by using the S2 and E2 potentials, but it could not be reproduced by using the E3 and S3 potentials; the three-body term acts repulsively at high temperatures to retain the O–Si–O bond angle at the tetrahedral angle in spite of many bonds being broken at such temperatures. Therefore, the three-body term is inappropriate for studying the properties and the structure of v-SiO2 at high temperature. The origin of the volume–temperature dependence was discussed based on the analysis of the structure.