High pressure band gap modification of LiCaAlF6

Abstract

First-principles density functional calculations together with experimental measurements demonstrate that pressure (uniform and uniaxial) increases the band gap of a perfect lithium hexafluoroaluminate (LiCaAlF6, LiCAF) crystal. As fluoride crystals can be highly transmitting at vacuum ultraviolet wavelengths, crystal modifications that further increase the band gap are highly sought after for future Vacuum ultraviolet applications. Through an extensive series of density functional theory simulations, we demonstrate that the band gap increases monotonically from 12.2 eV to 14.1 eV with the application of uniform pressure. Through joint theoretical and experimental investigation, we explore different uniaxial compressions that can be achieved through cutting-edge laser-shock compression. We find that uniaxial pressure also increases the LiCAF band gap by 0.3 and 0.4 eV for a- and c-axis compressions, respectively.