Refractive index of lithium fluoride for the solid and liquid phases upto 900GPa

Single crystal [100]-oriented lithium fluoride (LiF) is the most common window material used in dynamic compression research, making understanding its optical properties under compression important for advancing high pressure science. The transparency of LiF at high pressures is due to its mechanical properties and because it has the largest bandgap of any known transparent material, and LiF has been shown to remain optically transparent through the shock induced melt transition at 180GPa. While empirical fits to the measured density dependence of the refractive index have been provided, a model valid over the entire pressure range of experiments and based on a polarization-based theoretical description is lacking. We present a refractive index model for dynamically compressed LiF based on the Lorentz-Lorenz equation, where the molecular polarizability is determined using a single oscillator model with a strain polarizability parameter Λ = 0.73. We show that our modeling approach provides an excellent match to the LiF refractive index data for both shock compression experiments through the shock induced melting and ramp compression experiments where LiF remains solid upto 900 GPa (density compression greater than 3-fold).