Carrier Self-trapping and Luminescence in Intrinsically Activated Scintillator: Cesium Hafnium Chloride (Cs2HfCl6)

Abstract

There has been renewed interest in the scintillation properties of Cs2HfCl6 (CHC) primarily due to its favorable characteristics such as nonhygroscopicity, cubic structure, and fairly high light yield without any intentional doping. In this work, we report on its electronic and optical properties using first-principles calculations. As a large gap insulator (band gap ∼ 6 eV), CHC favors the formation of localized charge carriers, viz., Vk centers and electron polarons. The [HfCl6]-octahedra play a central role in trapping both types of localized carriers that leads to several low energy excitonic structures or self-trapped excitons (STEs). The observed emission spectrum of CHC is compared with our modeled STE structures and their emission energies. We find that Zr present as an unintentional impurity has high solubility in CHC and may be responsible for a secondary emission peak observed around 480 nm. Finally, we mention the electronic structure of the bromide and iodide analogues of CHC as well as the m...