The Influence of Halide Ion Substitution on Energy Structure and Luminescence Efficiency in CeBrI and CeBrI Crystals.

This study aims to determine the optimum composition of the CeBrI compound to achieve the maximum light output. It is based on calculations of the band energy structure of crystals, specifically taking into account the characteristics of the mutual location of local and band 5d states of the Ce ions. The band energy structures for CeBrI and CeBrI crystals were calculated using the projector augmented wave method. The valence band was found to be formed by the hybridized states of 4p Br and 5p I. The 4f states of Ce are located in the energy forbidden band gap. The conduction band is formed by the localized 5d1 states, which are created by the interaction between the 5d states of Ce and the 4f hole of the cerium ion. The higher-lying delocalized 5d2 states of Ce correspond to the energy levels of the 5d states of Ce in the field of the halide Cl (Br) hole. The relative location of 5d1 and 5d2 bands determines the intensity of 5d-4f luminescence. The bottom of the conduction band is formed by localized 5d1 states in the CeBrI crystal. The local character of the bottom of the conduction band in the CeBrI crystal favors the formation of self-trapped Frenkel excitons. Transitions between the 5d1 and 4f states are responsible for 5d-4f exciton luminescence. In the CeBrI crystal, the conduction band is formed by mixing the localized 5d1 and delocalized 5d2 states, which leads to quenching the 5d-4f luminescence and a decrease in the light output despite the decrease in the forbidden band gap. CsBrI is the optimum composition of the system to achieve the maximum light output.