Potassium-Impurity-Modulated Self-Trapped Excitons and Energy Transfer in Phase-Separated, Mn⁴⁺-Doped Li₂HfF₆

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, Mn⁴⁺-doped Li₂HfF₆ phosphors were synthesized by room-temperature coprecipitation to investigate impurity-modulated STEs and their role in Mn⁴⁺ sensitization. High-resolution neutron powder diffraction reveals impurity-associated phase separation into two closely related Li₂HfF₆ lattices. Photoluminescence and cathodoluminescence measurements resolve two broad host-emission components: STE1, assigned to intrinsic Li₂HfF₆, and redshifted STE2, associated with a K-containing lattice. Under high-energy cathodoluminescence excitation, enhanced Mn⁴⁺ red emission correlates with STE2 emission but not with STE1 emission, supporting a selective STE2 → Mn⁴⁺ 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 Mn⁴⁺ luminescence, establishing impurity modulation as an effective strategy for tuning excitonic emission and host-to-activator energy transfer in complex fluoride phosphors.