The Refractive Index and Transparency of Lithium Fluoride Compressed to 800~GPa*

Lithium fluoride, ramp compressed by direct laser ablation, is observed to remain transparent up to 800 GPa. Simultaneous measurements of the free-surface and interface (particle) velocities in a two-section diamond-LiF target determine the velocity-correction factor and the refractive index of compressed LiF. The refractive index is observed to increase linearly with density over pressures of 30 to 800 GPa. An effective single-oscillator model shows that the refractive index is linear in density as a result of the optical gap closing monotonically with increasing density. Extrapolation of these results indicate that metallization of LiF should occur at pressures significantly higher than the Goldhammer--Herzfeld criterion ($\sim$ 2750 GPa), suggesting that LiF will remain transparent at extremely high pressures.