Mechanical strength and coordination defects in compressed silica glass: Molecular dynamics simulations

Contrary to ordinary solids, which are normally known to harden by compression, the compressibility of $\mathrm{Si}{\mathrm{O}}_{2}$ (silica) glass has a maximum at about $2--4\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and its mechanical strength shows a minimum around $10\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. At this pressure, the compression of silica glass undergoes a change from purely elastic to plastic, and samples recovered from above $10\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ are found to be permanently densified. Using an improved, ab initio parametrized interatomic potential for $\mathrm{Si}{\mathrm{O}}_{2}$ we provide here a unified picture of the compression mechanisms based on the pressure-induced appearance of unquenchable fivefold defects. By means of molecular-dynamic simulations we find them to be responsible for the reduction of the mechanical strength and for permanent densification. We also find that the compressibility maximum does not require changes of the tetrahedral network topology.