High-pressure structural changes in liquid silica.

The structural properties of liquid silica at high pressure and moderate temperature conditions, also referred to as the warm dense matter regime, were investigated using time-resolved $K$-edge x-ray absorption spectroscopy and ab initio calculations. We used a nanosecond laser beam to compress uniformly a solid $\mathrm{Si}{\mathrm{O}}_{2}$ target and a picosecond laser beam to generate a broadband x-ray source. We obtained x-ray absorption spectra at the Si $K$ edge over a large pressure-temperature domain to probe the liquid phase up to 3.6 times the normal solid density. Using ab initio simulations, we are able to interpret the changes in the x-ray absorption near-edge structure with increasing densities as an increase in the coordination number of silicon by oxygen atoms from 4 to 9. This indicates that, up to significant temperatures, the liquid structure becomes akin to what is found in the solid $\mathrm{Si}{\mathrm{O}}_{2}$ phases.