High-excited-state splitting and multimode vibrational coupling in Mn⁴⁺-activated fluoride phosphor

Excited-states play a crucial role in the optical absorption and luminescence of solids and hence their accurate information is highly desired. Herein, we attempt to seize the excited-states information of Mn⁴⁺ ions in K₂SiF₆ microcrystals via measuring and calculating their variable-temperature photoluminescence excitation (PLE) spectra. At cryogenic temperatures, an unpredicted splitting of the high-excited-state is observed. Moreover, the two-split high-excited-state levels are further revealed to primarily couple with the two hyperfine split modes of quasi-localized v2 vibration in the distorted Mn-F₆ octahedral configuration, whereas the coupling strengths are found to be substantially different from each other. The slightly split vibrational mode is firmly supported by the low-temperature Raman spectra. Jahn-Teller lattice distortion is believed to be responsible for the observed splitting of the electronic high-excited-state and the quasi-localized vibrational mode.