The Index of Refraction of Lithium Fluoride at Pressures in Excess of 100 GPa

The compression of materials to high pressure can alter their optical properties in ways that provide insight into the resulting structural changes. Under strong shock compression; transparent insulators transform into conducting fluids as a result of pressure-induced reduction of the band gap and thermal promotion of electrons across that gap. A new ramp compression technique; direct-drive shaped ablation, is used to compress LiF to 800 GPa without generating shocks thereby producing high pressures at significantly lower temperatures than would be created by shock waves. In this study, ramp compressed lithium fluoride (LiF) is observed to remain transparent to 800 GPa, pressures seven times higher than previous shock compression experiments. The ramp compressed refractive index of LiF is measured at pressures up to 800 GPa and depends linearly on density over this range. This is the highest pressure refractive index measurement made to date.;The linear dependence of the refractive index and density is examined using a single-oscillator model. This model indicates that the linear behavior is a result of monoatomic closure of the band gap. Extrapolation of these results indicates that the band gap closure (metallization) will be greater than 5,000 GPa, well above the Goldhammer-Herzfeld criterion for metallization (∼ 2,860 GPa). The high metallization pressure of LiF is attributed to its large band gap and isoelectronic counterparts that exhibit high metallization pressures.;The high pressure transparency of LiF has technical utility as an optical window for materials studies since the transparency at high pressure allows in situ measurements of samples confined by that window. The observed transparency and measurement of LiF refractive index enables advancement of those experiments to higher pressure regimes.