A novel red phosphor Cs2NaGaF6:Mn4+ with ultra-strong zero-phonon lines and long wavelength phonon sidebands for high-quality WLEDs

Abstract

For high-efficiency lighting and wide color gamut backlight display, high-quality narrow-band red phosphors for WLEDs are still in high demand. Herein, a novel red-emitting fluoride phosphor Cs₂NaGaF₆:Mn⁴⁺ was successfully synthesized by a simple two-step co-precipitation method and exhibits ultra-intense zero-phonon lines (ZPLs) and long wavelength phonon sidebands under 468 nm blue light irradiation. The ZPL emission peak of Cs₂NaGaF₆:Mn⁴⁺ was located at 627 nm, which is much stronger than its υ₆ vibration peak, more matchable with the eye-sensitive region of humans, and beneficial for obtaining higher luminous efficiency of WLEDs. Interestingly, the υ₆ vibration peak of this red phosphor is at 636.5 nm, which is larger than that of the common fluoride phosphor A₂BF₆:Mn⁴⁺ (usually at about 630 nm, represented by K₂SiF₆:Mn⁴⁺) at about 6.5 nm. Thanks to the longer wavelength of the υ₆ vibration peak, the chromaticity coordinates (0.7026, 0.2910) with a larger x-coordinate value were realized, leading to a potentially wider color gamut of WLEDs. In addition, this phosphor has high thermal stability and its emission intensity at 423 K remains 93.7% of the initial intensity at room temperature. The lumen efficiency of WLED1 packaged with a mixture of Cs₂NaGaF₆:Mn⁴⁺ and YAG:Ce³⁺ on the InGaN blue chip is 115.7 lm W⁻¹ with the color temperature (T_c) = 3390 K and the colour rendering index (R_a) = 92.5 under 20 mA driving current. The chromaticity coordinates of WLED2 packaged with Cs₂NaGaF₆:Mn⁴⁺ and β-SiAlON:Eu²⁺ on the InGaN blue chip are (0.3149, 0.3262) and the calculated color gamut is up to 118.4% (NTSC). These results indicate that Cs₂NaGaF₆:Mn⁴⁺ red phosphors have promising applications in the high-quality lighting and display fields.