Novel Mn⁴⁺-activated fluoride red phosphors induced by alkaline metal ions replacement: Green synthesis, luminescence enhancement, high stability and application in warm WLED

Red-emitting Mn⁴⁺-doped fluoride phosphors are promising fluorescent material for use in high-performance warm white light emitting diodes (WLEDs). However, low stability and non-green synthesis method limit the development and commercialization of such phosphor. Herein, a series of novel (Li/Na/Cs)_0.4Ba_0.8SiF₆:Mn⁴⁺ phosphors with optimized fluorescence properties induced by alkaline metal ions replacement are successfully synthesized via a green and eco-friendly approach. Under the irradiation of 460 nm blue light, Cs_0.4Ba_0.8SiF₆:Mn⁴⁺ red phosphor with narrow band emission peak has the best optical performance. The optimal doping concentration of Mn⁴⁺ in Cs_0.4Ba_0.8SiF₆ host and the reason for concentration quenching are systematically analyzed. Furthermore, the moisture resistance and thermal stability of this phosphor are discussed. After being soaked in deionized water for 90 min, the PL intensity of Cs_0.4Ba_0.8SiF₆:Mn⁴⁺ phosphor can maintain at 40% of that in air, which is 4 times higher than commercial K₂SiF₆:Mn⁴⁺. At the operating temperature of LED (423 K), Cs_0.4Ba_0.8SiF₆:Mn⁴⁺ red phosphor can exhibit stable light output intensity and color. A warm WLED with low correlated color temperature of 4336 K, high color rendering index of 88.4 and high lumen efficiency of 105.96 lm/W is fabricated via using Cs_0.4Ba_0.8SiF₆:Mn⁴⁺ phosphor as a component of red-light, and the warm WLED can maintain stable output even at high driving currents. Overall, the alkali metal ion replacement and green synthesis strategies have opened up a new perspective for the synthesis of efficient Mn⁴⁺-doped fluoride phosphors, which has dual benefits for optimizing the performance of WLED and environmental protection.