Optical characteristics, morphology evolution and thermal stability of novel red-emitting Mn4+-activated K2LiAl1-yGayF6 solid solution phosphors for high-performance warm WLED

A series of novel narrow-band red-emitting K2LiAl1-yGayF6:xMn4+ (y = 0, 0.2, 0.4, 0.6, 0.8 and 1) (x = 0.02, 0.04, 0.06, 0.08 and 0.1) phosphors were synthesized using a simple coprecipitation reaction approach. The evolutions of crystal structure and morphology for Ga3+ and Mn4+ doped K2LiAlF6 phosphors have been systematically studied via X-ray powder diffraction (XRD) and scanning electron microscopy (SEM), respectively. The energy-dispersive X-ray spectroscopy (EDS) were conducted to investigate elemental composition and distribution of the phosphor in detail. The fluorescence properties of the phosphors at room temperature were investigated from excitation spectra, emission spectra and luminescence decay curves. According to the calculation of crystal field parameters (Dq, B and C), nephelauxetic ratio parameter (β1) and correlated color temperature as well as color purity of red light, it was proved that Mn4+ in K2LiAl0.4Ga0.6F6 host experienced a strong crystal field and could emit bright narrow-band red light around 636 nm owing to the spin-forbidden 2Eg → 4A2g transitions of Mn4+ with distinguished correlated color temperature and color purity. In addition, the concentration quenching behavior and energy-transfer mechanism of K2LiAl0.4Ga0.6F6:Mn4+ solid solution phosphors were explored and analyzed in detail. It was worth noting that this type of phosphor has an excellent thermal quenching behavior. Its fluorescence intensity can maintain about 74% at 423 K (150 °C) of its original value at room temperature. Importantly, employing the as-prepared K2LiAl0.4Ga0.6F6:0.04Mn4+ sample as a red component can dramatically improve the color rendering index from 75.3 to 88.1 and decrease the correlated color temperature from 6180 K to 3204 K of white light-emitting diodes (WLEDs). This work provided a new insight to explore novel Mn4+-activated red phosphors for warm WLEDs.