Growth, structure, and temperature dependent emission processes in emerging metal hexachloride scintillators Cs2HfCl6 and Cs2ZrCl6

Crystals of metal hexachlorides Cs2MCl6 (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 Cs2HfCl6 and Cs2ZrCl6 crystals in the broad temperature range from 9 to 300 K. X-ray diffraction data confirmed the same cubic structure (space group Fm3m) for Cs2HfCl6 and Cs2ZrCl6 over the entire examined temperature range. The room temperature scintillation light yield of Cs2HfCl6 excited with a Cs-137 gamma-source is measured to be 24 800 photons per MeV, while Cs2ZrCl6 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 alpha-particles from an Am-241 source is equal to 0.39 for Cs2HfCl6 and 0.35 for Cs2ZrCl6. 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.