Persistent luminescence induced by the introduction of multi-valent Mn ions in K₂LiBF₆ (B = Al, Ga and In) fluoride phosphors

Mn⁴⁺ – activated elpasolite fluorides K₂LiBF₆ (B = Al, Ga and In) are well-known red-emitting phosphors for white light emitting diodes (WLEDs) application. Such fluorides exhibit narrow emissions at around 630 nm under near ultraviolet (NUV) or blue excitations, but in this work, we introduced an unusually broad emission band in the green to orange region under NUV excitation at low temperature by introducing impurity to the elpasolite structure. The nature of this photoluminescence (PL) has been verified to be from Mn²⁺ species, so we observed the multi-valence of Mn ions (Mn²⁺ and Mn⁴⁺) in fluorides, which is rarely discussed in literatures. Besides, the influence of Mn²⁺ on Mn⁴⁺ was studied in samples with different Mn⁴⁺/Mn²⁺ ratios. The configuration-coordinate model and crystal theory were employed to help us to get a deeper understanding that the presence of Mn²⁺ affects the stretching vibration mode ν₂ of Mn⁴⁺ in octahedra. Furthermore, the thermal quenching of Mn²⁺ and Mn⁴⁺ was studied for ratiometric optical thermometer application. In addition, persistent luminescence (PersL) was found in the proposed fluorides. For example, Under NUV irradiation, white PL was achieved in K₂LiGaF₆: Mn, and the emission turned to the green once excitation ceased. We herein demonstrated a novel multi-valence synthetic method for dual-color emitting elpasolite fluorides and propose a mechanism for the observation of white PL and green PersL based on the multi-valence properties.