Structure and optical properties of Tl₂LiYCl₆: A new phosphor scintillator material for gamma ray and neutron detection

Tl based double halide perovskites are emerging as promising candidates for commercial scintillator applications such as gamma ray and neutron detection. They offer increased detection efficiency for applications such as gamma rays detection due to the increased stopping power with high Z Tl (Z=81) and good detection efficiency for thermal neutrons due to the presence of Li.

These materials can exhibit intrinsic activation due to the presence of the optically active Tl⁺ ions. We find that the effect of the weak Tl 6s - Cl 3p hybridization in Tl₂LiYCl₆ leads to a weakly disordered low temperature structure that from a theory point of view has better optical properties for scintillation than the ordered cubic double perovskite structure that was found experimentally to be stable above 463K. Specifically, we employ first-principles calculations to determine the unstable phonon modes in the cubic phase that allow us to determine the less ordered low temperature structure. We also present electronic structure studies for both the ground state and excited states of this system to show that it has good scintillation properties such as the suppression of trapping states typically found in other halide scintillator materials.