Refractive index of lithium fluoride ramp compressed to 800 GPa

We report the highest pressure under which a transparent insulator has been observed. The refractive index of ramp-compressed lithium fluoride (LiF) is measured up to a pressure of 800 GPa and is observed to maintain its linear dependence on density. An effective single-oscillator model infers that the bandgap monotonically closes with increasing density, indicating that metallization of LiF should occur at pressures above 4000 GPa, and that LiF should remain transparent at extremely high pressures.