Ab initio simulation of permanent densification in silica glass

To clarify the microscopic structure of densified $\mathrm{Si}{\mathrm{O}}_{2}$ glass, we have conducted ab initio molecular-dynamics simulations on the decompression process of $\mathrm{Si}{\mathrm{O}}_{2}$ glass in its relaxed state from high pressures up to 40 GPa. When decompressed from high pressures above at least 15 GPa, the density and structure always converge to those of densified glass, while the coordination number of silicon decreases to four rapidly. This is in good agreement with previous experimental studies and strongly suggests that densified glass behaves as a high-pressure polymorph of $\mathrm{Si}{\mathrm{O}}_{2}$ glass. In comparison to ordinary glass, although the coordination number of densified glass is almost the same, the size of an intermediate-range network consisting of $\mathrm{Si}{\mathrm{O}}_{4}$ tetrahedra is smaller. Detailed analyses clarify that $\mathrm{Si}{\mathrm{O}}_{4}$ tetrahedra in densified glass are deformed and the Si-O bonds are less covalent.