First-principles calculations of electronic and optical properties of LiCaAlF6 and LiSrAlF6 crystals as VUV to UV solid-state laser materials

We report the density functional calculations of the electronic and optical properties of perfect LiCaAlF6 (LiCAF) and LiSrAlF6 (LiSAF) crystals. The calculations are based on the Perdew-Burke-Ernzerhof (PBE) functional employing 35% exact exchange. Using optimized unit crystal volumes and equilibrium lattice constants, both LiCAF and LiSAF are found to have indirect band gaps of 12.23 and 11.79 eV, respectively. The band gap energies of these fluoride crystals are also observed to increase upon application of pressure by uniform volume compression. Moreover, their bulk moduli are determined to be 108.01 (LiCAF) and 83.75 GPa (LiSAF), while their static dielectric constants are 1.27 (LiCAF) and 1.26 (LiSAF). Considering the dielectric functions, refractive indices, and absorption coefficients, both perfect LiCAF and LiSAF crystals are viable vacuum ultraviolet (VUV) to ultraviolet (UV) laser host media. With knowledge of the different optical transitions and pressure dependence, our results yield helpful insights on the use of these fluoride compounds as effective solid-state laser materials in the VUV region.