Ab initiomany-body Green's function calculations of optical properties of LiF at high pressures

Abstract

We present density functional theory (DFT) $+$ quasiparticle self-energy $({G}_{0}{W}_{0})+$ Bethe-Salpeter calculations of the real and imaginary parts of the long-wavelength dielectric function of LiF between ambient pressure and $P=5$ Mbars. While the optical absorption spectrum is predicted to show dramatic pressure-dependent features above the optical gap, the index of refraction well below the gap is shown to exhibit the same trends as that seen in both DFT calculations and experiment: a roughly linear increase with density. This increase does not result from a decrease in the band gap, but rather follows from the increase in oscillator strength which counteracts a smaller increase in band gap with $P$. Our calculations also suggest that the index of refraction (for visible and near-UV light) of the higher-$T\phantom{\rule{4pt}{0ex}}{B}_{2}$ phase should be quite close to that of the ${B}_{1}$ (ambient crystalline) phase. These findings may be of interest to researchers who use LiF as a window material in dynamic compression experiments.