Potassium‐Impurity‐Modulated Self‐Trapped Excitons and Energy Transfer in Phase‐Separated, Mn 4+ ‐Doped Li 2 HfF 6

ABSTRACT Self‐trapped excitons (STEs) play an important role in the luminescence of halide materials, yet their formation and manipulation in complex fluorides remain largely underexplored. Here, Mn 4+ ‐doped Li 2 HfF 6 phosphors were synthesized by room‐temperature coprecipitation to investigate impurity‐modulated STEs and their role in Mn 4+ sensitization. High‐resolution neutron powder diffraction reveals impurity‐associated phase separation into two closely related Li 2 HfF 6 lattices. Photoluminescence and cathodoluminescence measurements resolve two broad host‐emission components: STE1, assigned to intrinsic Li 2 HfF 6 , and redshifted STE2, associated with a K‐containing lattice. Under high‐energy cathodoluminescence excitation, enhanced Mn 4+ red emission correlates with STE2 emission but not with STE1 emission, supporting a selective STE2 → Mn 4+ energy‐transfer pathway. Density functional theory ΔSCF (self‐consistent field) calculations yield distinct relaxed STE1 and STE2 configurations, confirming the K‐induced redshift of the STE emission. These results identify K‐associated STE2 as a key sensitizing state for Mn 4+ luminescence, establishing impurity modulation as an effective strategy for tuning excitonic emission and host‐to‐activator energy transfer in complex fluoride phosphors.