Utilizing the Unique Redox Reaction Between Transition Metal Ions to Improve Mn⁴⁺ Doping Concentration and Achieve a High-Performance Red-Emitting Cs₂NbF₆: Mn⁴⁺ Phosphor

Synthesis of a Mn4+-activated fluoride red-emitting phosphor combining high efficiency, excellent thermal stability, and outstanding moisture resistance still remains a challenge. Herein, this work synthesizes a red-emitting phosphor Cs2NbF6: Mn4+ (CNF: Mn4+) based on the cation-exchange method. Interestingly, as a result of the redox reaction between Mn3+ (Mn2+) and Nb5+ ions during the exchange reaction, the real Mn4+ concentration is remarkably increased, while Nb5+ ions are reduced to Nb4+. The obtained optimum CNF: Mn4+ phosphor shows high internal and external photoluminescence (PL) quantum yield (QY) of 92% and 58%, respectively. Moreover, the CNF: Mn4+ exhibits excellent thermal stability (I-@150 = 105%) and moisture resistance. A white light-emitting diode (LED) packaged by combining blue diode chips, Y3Al5O12:Ce3+ yellow phosphor, and CNF: Mn4+ red phosphor demonstrates a low correlated color temperature (CCT) of 3872 K, high color rendering index (CRI) of Ra = 88, and a luminous efficacy of 105 lm W-1. The high-performance of the white LED suggests the great potential of the CNF: Mn4+ narrowband red phosphor in display and lighting fields.