First-principles optical response of shock-compressed LiF: Quasiparticle, excitonic, and ionic-temperature effects

Abstract: We investigate the refractive index $n$ of LiF using DFT+G$_0$W$_0$+BSE, with ionic-temperature effects included through QMD. We calculate photon-energy dispersions $n(ω)$ and $k(ω)$ at ambient pressure and $n(ρ,T)$ at 532 and 1550 nm under shock compression, where $ρ$ and $T$ vary together. Quasiparticle band structures at ambient and compressed conditions and the ambient-pressure orbital-projected density of states connect the optical response to the electronic structure. At ambient pressure, G$_0$W$_0$ yields a quasiparticle gap of 14.25 eV, close to the experimental 14.2 eV, while BSE reproduces the main excitonic feature at 12.5 eV, close to the observed 12.6 eV. Quasiparticle and excitonic effects are thus essential for accurate optical dispersion; the hybrid HSE functional does not reproduce the optical spectra. Along the principal Hugoniot up to 140 GPa, $n(ρ)$ agrees closely with shock data at 1550 nm and acceptably at 532 nm, and agrees better overall than earlier first-principles calculations. Above approximately 110-120 GPa, QMD produces a downturn relative to cold $n(ρ)$ curves, suggesting that ionic dynamics may contribute to deviations from the linear $n$-$ρ$ Gladstone-Dale relation. QMD-sampling and finite-k-grid uncertainties are estimated, and transition-peak broadening is assessed using a recently proposed criterion. The gap increases under pressure, with a transition to a $Γ\to\mathrm L$ indirect gap near 50 GPa. An exploratory calculation at approximately 1400 GPa finds a gap of about 24 eV, disfavoring gap-closure metallization. Structural and elastic benchmarks of the underlying models are also provided.