Discovery of New Compounds and Scintillators of the A4BX6 Family: Crystal Structure, Thermal, Optical, and Scintillation Properties

We report the discovery of four new compounds of Eu2+-doped scintillators [Cs3Rb]­SrI6, [Cs3K]­SrI6, [Cs3Rb]­CaI6, and [Cs3K]­CaI6. A compositional engineering strategy dictated by the structural systematics of the base compounds Cs4CaI6 and Cs4SrI6 (space group R3̅c-hexagonal basis) was conducted to confirm an ideal solid solution for this system. Substitution of smaller monovalent cations Rb1+ and K1+ for Cs1+, maintained the crystal structure, producing an ordered substitution. Crystals were initially grown doped with 0.5% Eu to compare scintillation properties, and a Eu2+ optimization study determined that 7% Eu was the ideal concentration. A 56 mm3 crystal of composition [Cs3.5Rb0.5]­SrI6:Eu 7% gave the best measured scintillation performance with an absolute light yield of 75 000 ph/MeV and energy resolution of 3.3% at 662 keV.