Direct band gap tunability of the LiYF4 crystal through high-pressure applications

The pressure dependence of the electronic band structure and corresponding optical properties for a perfect LiYF4 crystal are investigated through density functional theory. For the optical properties, self-energy corrections are included using the Green's function-based GW approximation. At zero pressure, LiYF4 is found to have a direct band gap of 11.1 eV originating from a transition between the fluorine 2p valence band maximum and the yttrium 5d conduction band minimum. The band gap increases uniformly upon volume compression without any direct-to-indirect band gap transition while flattening out at pressures >15 GPa. Band gap tuning through uniaxial compression is more efficient when pressure is applied along the crystallographic c-axis. Our results demonstrate that the ionic LiYF4 crystal is a potential vacuum ultraviolet (VUV) laser host material with band gap energy and absorption edge that can effectively be controlled through high pressure application.