Molecular dynamics simulation of structural transformation in SiO2 glass under densification

Molecular dynamics simulation of silica glass has been carried out to investigate the structural transformation in high-pressure densification. The fractions of SiOx (x = 4, 5, and 6) structural units change with the change of density. We found that only corner-sharing bond (one common oxygen atom) appears in the SiO4-SiO4 connectivity, whereas both of corner- and edge-sharing (two common oxygen atom) bonds appear in the SiO5-SiO5 connectivity and three types (corner-, edge- and face- (three common oxygen atom)) of sharing bonds appear in the SiO6-SiO6 connectivity. The first peak splitting of the pair radial distribution function (RDF) G(si-si)(r) is caused by Si atoms linked together via edge- and face-sharing bonds. The new second peak of the pair RDF G(o-o)(r) at the high density originates from oxygen atoms of the edge-sharing bonds. Void analysis indicated that a significant number of O-voids, in which void contacted with four oxygen atoms, exist in the samples. The biggest voids belong to these O-voids. The volume of O-voids changes slightly whereas one of total voids decreases quickly with increasing density. We also found that the density of each SiOx unit type change under high-pressure densification and the density of silica glass can be expressed via the fraction and partial density of SiOx units. Oxygen atoms in the form of hexagonal close packed (hcp) structure are found in the sample of 3.96 g.cm(-3). Both of oxygen hcp and faced centered cubic (fcc) clusters are found in the sample with the density above 3.96 g.cm(-3).