Band Gap Calculations for Thorium‐Doped LiCAF

Abstract

The large band gap insulator LiCaAlF₆ (LiCAF) has been proposed as a possible host crystal for future realizations of a solid-state based thorium-229 nuclear clock, due to its excellent optical transmission in the vacuum ultraviolet range. To enable direct optical manipulation of the thorium isomeric state, the band gap has to remain larger than the nuclear excitation energy upon crystal doping. Here, a systematic search for possible charge compensation mechanisms, defect locations, and the emergence of other compounds, using density functional theory, is presented. Out of 535 optimized structures, the energetically most favorable arrangement is Th∙Al+V′Li with an estimated band gap of 11.4 eV. Evaluating relevant uncertainties of the used methods suggests that the doped material remains transparent at the ²²⁹Th isomer energy for all investigated configurations.