Zero-dimensional (0D) halides with the general formula A 2 BX 6 (such as Cs 2 HfI 6 ) have emerged as a promising class of next-generation scintillator materials characterized by their high luminescence efficiency. This study investigated the novel 0D halide Li 2 HfI 6 , where A-site Li + substitution induces a structural transition to a rhombohedral structure. Comprehensive experimental and density functional theory (DFT) analyses reveal that Li + substitution facilitates carrier delocalization. Temperature-dependent photoluminescence identified a shallow 17 meV de-trapping barrier from the self-trapped exciton (STE) to the free exciton (FE) state. This thermally induced transition, paralleling phenomena well-observed in 2D perovskites, results in a coexistence of high-energy FE and low-energy STE emissions at room temperature. The emergence of the FE component enables a rapid scintillation decay time of less than 100 ns. In addition, Li 2 HfI 6 achieves a high thermal neutron light yield of 17,000 photons/neutron, which is approximately three times higher than commercial 6 Li-glass. These characteristics demonstrate that Li 2 HfI 6 is a promising candidate for 3 He-alternative technologies. This work establishes A-site cation engineering as an effective design principle for controlling the structure and luminescence properties of 0D halide neutron scintillators. • Novel Li 2 HfI 6 scintillator single crystals were grown using the Bridgman method. • Li 2 HfI 6 exhibits free-exciton emission at room temperature. • Li 2 HfI 6 shows a light output of 17,000 photons/n th . • A fast decay time (< 100 ns) was observed owing to free-exciton emission.
Fujiwara et al. (2026) studied this question.