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

Abstract Synthesis of a Mn 4+ ‐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 Cs 2 NbF 6 : Mn 4+ (CNF: Mn 4+ ) based on the cation‐exchange method. Interestingly, as a result of the redox reaction between Mn 3+ (Mn 2+ ) and Nb 5+ ions during the exchange reaction, the real Mn 4+ concentration is remarkably increased, while Nb 5+ ions are reduced to Nb 4+ . The obtained optimum CNF: Mn 4+ phosphor shows high internal and external photoluminescence (PL) quantum yield (QY) of 92% and 58%, respectively. Moreover, the CNF: Mn 4+ exhibits excellent thermal stability ( I @150 = 105%) and moisture resistance. A white light‐emitting diode (LED) packaged by combining blue diode chips, Y 3 Al 5 O 12 :Ce 3+ yellow phosphor, and CNF: Mn 4+ 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: Mn 4+ narrowband red phosphor in display and lighting fields.