Fundamental properties of novel zero-dimensional halide Na₂HfI₆ and application for neutron detection by Li doping

Zero-dimensional (0D) metal halide scintillators have attracted considerable attention owing to their exceptional emissive properties derived from strongly localized self-trapped excitons.In this paper, we report the development of a novel thermal neutron scintillator-Na2-xLixHfI6 (0 x 2)-achieved by strategically substituting lithium for the Na of the 0D Na2HfI6 host.The Na2HfI6 host material has a 0D structure with an [HfI6] 2-octahedral unit, exhibiting broad red-to-near-infrared (NIR) emission with a large Stokes shift, driven by strong electron-phonon coupling (S = 56.7)and high exciton binding energy (Eb = 200 meV).Density functional theory calculations revealed that the A site cations (Na + and Li + ) did not contribute to the valence or conduction band edges.This indicated that the fundamental electronic structure was preserved upon Li substitution; consequently, the Li-alloyed Na2-xLixHfI6 also exhibited the characteristic broad red-to-NIR emission.Remarkably, the Na1.5Li0.5HfI6crystal demonstrated a light yield of 37,000 ph./nth under thermal neutron irradiation, approximately six times higher than that of the standard 6 Li glass scintillator (GS20).With the fast decay time (650 ns) as red/NIR emission, high light output, and red/NIR emission wavelength, Na2-xLixHfI6 offers a robust and high-performance alternative material to conventional scintillators.