The structure of silica glass (SiO2) at different densities and at temperatures of 500 K is investigated by molecular dynamics simulation. Results reveal that at density of 3.317 g/cm 3 , the structure of silica glass mainly comprises two phases: SiO4-and SiO5-phases. With the increase of density, the structure tends to transform from SiO4-phase into SiO6-phase. At density of 3.582 g/cm 3 , the structure comprises three phases: SiO4-, SiO5-, and SiO6-phases, however, the SiO5-phase is dominant. At higher density (3.994 g/cm 3 ), the structure mainly consists of two main phases: SiO5-and SiO6-phases. In the SiO4phase, the SiO4 units mainly link to each other via corner-sharing bonds. In the SiO5-phase, the SiO5 units link to each other via both corner-and edge-sharing bonds. For SiO6-phase, the SiO6 units can link to each other via corner-, edge-, and face-sharing bonds. The SiO4-, SiO5-, and SiO6-phases form SiO4-SiO5-and SiO6-grains respectively and they are not distributed uniformly in model. This results in the polymorphism in the silica glass at high density.

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The structure of the SiO2 system consisting of 12800 atoms is performed via molecular dynamics simulation with the Tersoff potential. Our simulation shows that the onset of the melting temperature is at 3450 K. This value is much higher than the previous result of Ringdalen and co-workers [26]. The structural evolution of the system is analyzed through the pair radial distribution function, the distribution of the bond length and the distribution of the bond angle. Structural parameters are compared with the previous experimental and simulated results. The mean square displacement is also discussed in this paper.

The structural properties of xAl2O3(1-x)SiO2 liquid systems have been investi-gated by molecular dynamics simulation in a wide range of compositions, x = 0.05–0.7 at 3000 K. The structure of liquid aluminosilicate system is clarified by analyzing the simplex and shell-core. The simulated results showed that the liquid consists of a large quantity of void-simplex, O-simplex, T-simplex, and SC-particle. Our simulation reveals that the densification of the liquid is due to the fact that the number of large simplexes and the radii of simplexes and SC-particles decrease. Besides, results also indicated that the distribution of cations in the liquid is not uniform.