Mn4+-Doped fluoride red phosphors with excellent moisture-resistant properties and high emission efficiency are highly desired for warm white LEDs and wide gamut liquid crystal displays. Herein, we synthesized a K2NaGaF6:Mn4+ red phosphor through a co-precipitation method at room temperature. The crystallographic structure of the K2NaGaF6 host was determined based on single crystal X-ray diffraction data for the first time. A high internal photoluminescence quantum yield of 82% was achieved for the 1.81 at% K2NaGaF6:Mn4+ sample. Through low-temperature and high-resolution spectroscopic analysis, we revealed that Mn4+ ions occupy at least four distinct sites in the K2NaGaF6 crystal. One of them is assigned to the Ga3+ site with high Oh site symmetry and the other three ones possess low non-centrosymmetric site symmetries. By employing 5.51 at% K2NaGaF6:Mn4+ as a red phosphor, a warm white LED with a low correlated color temperature of 3760 K, a high color rendering index of 92 (Ra) and a luminous efficacy of 156 lm W−1 was packaged. Aging tests at 85 °C and 85% relative humidity on the fabricated LEDs demonstrated that K2NaGaF6:Mn4+ has a better moisture-resistant property than that of the popular K2SiF6:Mn4+ phosphor. These findings demonstrate the great potential of K2NaGaF6:Mn4+ as a highly efficient red phosphor with excellent moisture-resistant performance.
A novel ternary-alkaline red emitting fluoride phosphor K2NaGaF6:Mn4+ was successfully synthesized through co-precipitation method. The crystal structure, morphology, electronic band structure and luminescence properties of K2NaGaF6:Mn4+ phosphors were investigated in details by using Rietveld refinement of X-ray diffraction data, scanning electron microscopy (SEM), density functional theory (DFT) calculation and different reaction parameters. The K2NaGaF6 host has a cubic unit cell with the space group Fmm and lattice parameters of a = 8.2577 (4) Å, Z = 4, Vcell = 563.08 (8) Å3. Under blue light excitation, Mn4+ activated K2NaGaF6 exhibits a bright narrow-band red emission. The PL properties of the K2NaGaF6:Mn4+ red phosphors were optimized with different Mn4+ concentrations and aging times. A warm-white LED device was fabricated using a blue LED chip combined with commercial yellow YAG:Ce3+ phosphor and synthesized K2NaGaF6:Mn4+ red phosphor. The color rendering index (CRI, Ra = 81.6) and corresponding color temperature (CCT = 3643 K) easily reached the commercial warm white light LED standards (Ra > 80 and CCT < 4000 K). All these results indicate that K2NaGaF6:Mn4+ phosphor would be a suitable red phosphor candidate for warm-white LED applications.
Chunyan Jiang,Mikhail G. Brik,Lihua Li,Liyi Li,Jie Peng,Jianan Wu,Maxim S. Molokeev,Ka-Leung Wong,Mingying Peng
Recently, as a key red component in the development of warm white light-emitting diodes (WLEDs), Mn4+-doped fluorides with narrow red emission have sparked rapidly growing interest because they improve color rendition and enhance the visual energy efficiency. Herein, a red nanophosphor, K2NaGaF6:Mn4+, with a diameter of 150–250 nm has been synthesized using a simple co-precipitation method. Rietveld refinement reveals that it crystallizes in the space group Fmm with the cell parameter a = 8.25320(4) Å. The exchange charge model (ECM) has been used to calculate the energy levels of Mn4+ ions in K2NaGaF6, which match well with the experimental spectra. The as-synthesized phosphor exhibits a narrow red emission at around 630 nm (spin-forbidden 2Eg → 4A2 transition of Mn4+ ions) when excited at 365 nm (4A2g → 4T1g) and 467 nm (4A2g → 4T2g), with a quantum efficiency (QE) of 61% and good resistance to thermal quenching. Based on the structure, the formation mechanism of ZPL has been discussed. In addition, the concentration-dependent decay curves of Mn4+ in K2NaGaF6 were fitted using the Inokuti–Hirayama model, suggesting that the dipole–dipole interactions determine the concentration quenching. Finally, encouraged by the good performance, a warm LED with a CRI of 89.4 and CCT of 3779 K was fabricated by employing the title nanophosphor as the red component. Our findings suggest that K2NaGaF6:Mn4+ can be a viable candidate for the red phosphor used in warm WLEDs.
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