First principle quantum description of the energetics associated with LaBr3, LaCl3, and Ce doped scintillators

Considerable interest is given to the excellent scintillation properties of cerium doped lanthanum chloride (LaCl₃) and lanthanum bromide (LaBr₃). The scintillation efficiencies are much greater than other materials, even those containing cerium. This high efficiency is attributed to the high mobility of electrons and holes, unique placement of the cerium 5d states within the band gap, and energy of the band gap. To better understand the scintillation process and better define the nature of the self trapped exciton (STE) within these unique scintillation materials, density functional theory (DFT), and Ab-inito (HF-MP2) calculations are reported. DFT calculations have yielded a qualitative description of the orbital composition and energy distribution of the band structure in the crystalline material. MP2 and single configuration interaction calculations have provided quantitative values for the band gap and provided energies for the possible range of excited states created following hole and electron creation. Based on this theoretical treatment, one possible description of the STE is the combination of V_k center (Br₂ ^-1) and LaBr⁺¹ species that recombine to form a distorted geometry LaBr₃* (triplet state). Depending on the distance between the LaBr and Br₂, the STE emission band can be reproduced.