Pressure-Induced Structural Changes in Liquid $\mathrm{S}\mathrm{i}{\mathrm{O}}_{\mathrm{2}}$ from $\mathrm{\text{Ab Initio}}$ Simulations

First-principles molecular dynamics simulations at constant pressure have been used to investigate the mechanisms of compression of liquid $\mathrm{S}\mathrm{i}{\mathrm{O}}_{\mathrm{2}}$. Liquid $\mathrm{S}\mathrm{i}{\mathrm{O}}_{\mathrm{2}}$ is found to become denser than quartz at a pressure of about 6 GPa, in agreement with extrapolations of lower pressure experimental data. The high compressibility of the liquid is traced to medium-range changes in the topology of the atomic network. These changes consist in an increase of network connectivity caused by the pressure-induced appearance of coordination defects.

Figure 1

Equations of state of quartz and liquid ${\mathrm{S}\mathrm{i}\mathrm{O}}_2$ at 3500 K. Diamonds: liquid (solid diamonds: run #1; open diamonds: run #2); gray circles: quartz. Dashed line: equation of state of the liquid at 2000 K, from experiments [3]. Inset: Volumes scaled with the average value of $R_{\mathrm{S}\mathrm{i}\mathrm{-}\mathrm{S}\mathrm{i}}^3$.

Figure 2

Percentage of Si atoms coordinated to 4, 5, and 6 oxygen atoms (black, gray, light gray, respectively). (a) ab initio simulations (solid symbols: run # 1, open symbols: run # 2); (b) simulations with the BKS force field [13, 25]. Lines are a guide to the eye.

Figure 3

Schematic diagrams of three different mechanisms of densification in liquid ${\mathrm{S}\mathrm{i}\mathrm{O}}_2$. Only Si atoms are shown. Pressure increases from up to down. (a) Reduction of the Si-Si distance, as in quartz; (b) elimination of three-membered rings [10]; (c) increase in connectivity caused by coordination defects.