• • Ce:LGCSB single crystals achieve a scintillation decay time of 1.23 ns with a 94.6% fast-component contribution and no detectable slow component, enabling sub-2-ns timing resolution for high-rate X-ray imaging and TOF-PET.
• • The high-entropy design induces severe lattice distortion that localizes Frenkel excitons, suppressing diffusion and accelerating recombination, as confirmed by first-principles calculations and transient dynamics.
• • The material maintains high light yield despite low Ce doping levels, demonstrating that entropy engineering can decouple decay time from light yield loss, a critical trade-off in conventional scintillators.
• • The proposed strategy of extending sluggish diffusion effects to the excitonic scale offers a general design framework for ultrafast scintillators, potentially applicable to other halide and oxide systems.