Growth, structure, and temperature dependent emission processes in emerging metal hexachloride scintillators Cs 2 HfCl 6 and Cs 2 ZrCl 6 .

Crystals of metal hexachlorides Cs 2 MCl 6 (M = Hf or Zr) have recently emerged as promising materials for scintillation applications due to their excellent energy resolution. In this work, we investigated the crystal structure and scintillation properties of Cs 2 HfCl 6 and Cs 2 ZrCl 6 crystals in the broad temperature range from 9 to 300 K. X-ray diffraction data confirmed the same cubic structure (space group Fm3m ) for Cs 2 HfCl 6 and Cs 2 ZrCl 6 over the entire examined temperature range. The room temperature scintillation light yield of Cs 2 HfCl 6 excited with a 137 Cs γ-source is measured to be 24 800 photons per MeV, while Cs 2 ZrCl 6 exhibits 33 900 photons per MeV resulting in energy resolutions of 5.3% and 4.5%, respectively. The alpha-to-beta ratio determined at room temperature for 5.5 MeV α-particles from an 241 Am source is equal to 0.39 for Cs 2 HfCl 6 and 0.35 for Cs 2 ZrCl 6 . The measurements of scintillation decay curves revealed complex kinetics due to delayed recombination processes. A tangible enhancement of the scintillation yield with heating is observed in the 125-150 K range. This effect is a manifestation of negative thermal quenching explained by thermal activation of trapped carriers. A model of the emission centre is proposed that consistently explains the observed changes of emission intensity with temperature in the crystals under study.