Exploring the Cs2LiLuCl6 elpasolite scintillator for thermal-neutron detection: A comprehensive DFT and TD-DFT studies

In this study, we investigate the halide perovskite material ${\mathrm{Cs}}_2{\mathrm{LiLuCl}}_6$ using both density functional theory (DFT) and time-dependent density functional theory (TD-DFT) approaches. Our analysis focuses on the structural, electronic, and optical properties of the material, conducted with the Quantum Espresso code. In order to confirm the structural stability of the material, we performed various tests including analysis of elastic constants, formation energy, tolerance factor, and phonon dispersion. Employing the generalized gradient approximation (GGA) and GGA+U, we determine that the material behaves as an insulator with a direct band gap of 5.3042 eV and 5.4245 eV, respectively. Additionally, DFT+U calculations were performed to study the dielectric function, reflectivity, absorption coefficient, and refractive index, revealing a high transmittance rate exceeding 90%. Under ideal conditions, the upper light yield (LYsc) is estimated to be $75411$ ph/MeV and $73739$ ph/MeV for the DFT and DFT+U methods, respectively. These results, along with the presence of lithium in ${\mathrm{Cs}}_2{\mathrm{LiLuCl}}_6$, highlight its potential for use in scintillation-based thermal-neutron detection applications.