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.

Figure 1

Density-temperature conditions inferred from the shock timing and velocity SOP and VISAR data coupled to the hydrodynamic simulations along the isotherms 1 eV (in blue) and 3 eV (in red). Highly compressed $\mathrm{Si}{\mathrm{O}}_2$ states were probed from either side of the Hugoniot. The green dots represent the conditions at which the ab initio simulations were performed. The orange lines represent the boundaries of the phase diagram proposed in Ref. [13].

Figure 2

Si $K$-edge XANES spectra measured (left) and calculated (right) near 1 eV (up) and 3 eV (down).

Figure 3

Shift of the first maximum position (blue circles) and foot $K$-edge position (red squares) as a function of density. Plain symbols are experimental data, and empty symbols are calculations. Left: results along the isotherm at 1 eV; right: results along the isotherm at 3 eV.

Figure 4

(a) Variation with density of the Si-O pair correlation function along the 3-eV isotherm given by the molecular dynamics simulations. (b) Variation of the coordination number for all the simulated conditions. Each color is associated with a temperature. (c) Energy shift of the energy of the $1s$ orbital with the density in the solid phases (dashed lines) and in the liquid (solid lines). The temperatures are indicated in the graph.