Structure analysis and luminescence properties of red-emitting iodide scintillators with potassium-hexachloroplatinate(IV) type structure

Halide compounds with chemical formula A2MX6 (A: alkali metal or Tl, M: tetravalent metal, X: halogen) have previously been reported as intrinsic scintillators. However, some application fields e.g., utilizing waveguides, require novel scintillators with emission wavelengths longer than that of Cs2HfI6 as bright and red/infrared scintillators. Thus, in this study, cation-substituted Cs2HfI6 crystals (Rb2HfI6, K2HfI6, Cs2ZrI6, Rb2ZrI6, and K2ZrI6) were grown using the vertical Bridgman method, and their crystal structures were analyzed via single crystal X-ray diffraction. The crystal structures of Cs2HfI6 and Cs2ZrI6 were determined to be cubic (space group: Fm3m), Rb2HfI6 and Rb2ZrI6 were tetragonal (space group: P4/mnc), and K2HfI6 and K2ZrI6 were monoclinic (space group P21/c). In addition, the emission spectra of Rb2HfI6, K2HfI6, Rb2ZrI6, and K2ZrI6 showed that their peaks exhibited a 25 nm red-shift in comparison with that of Cs2HfI6, whereas those of Cs2ZrI6 did not. For A2MI6 compounds, the emission was effectively determined by A-site cation substitution. Moreover, the pulse-height spectra were successfully measured for A2HfI6. The scintillation light outputs of Cs2HfI6, Rb2HfI6, and K2HfI6 were estimated to be approximately 60,000, 40,000, and 33,000 photons/MeV, respectively.