Synthesis and optical properties of highly efficient Na3KMg7(PO4)6:Eu2+ blue phosphor for full-spectrum white-light-emitting diodes: The role of Li2CO3 flux

Full-spectrum phosphor-converted white-light-emitting diodes (pc-WLED) are emerging as a mainstream technology in semiconductor lighting. Nevertheless, high-performance blue phosphor which can be excited efficiently by a 400 nm NUV diode chip is still lacking. Herein, we present a blue-emitting Na 3 KMg 7 (PO 4 ) 6 :Eu 2+ phosphor synthesized by the solid-reaction method. Particularly, we find that the using of Li 2 CO 3 as flux can significantly improve the crystal quality and thus the emission efficiency of the phosphor. Meanwhile, the excitation peak of the phosphor shifts from 365 to 400 nm, which is pivotal for efficient NUV (400 nm) diode chip excitation. The practical Eu 2+ concentration is also enhanced by using Li 2 CO 3 as flux, and the absorption efficiency is greatly increased. This phosphor exhibits superior PL thermal stability, namely retains 94% integrated photoluminescence intensity at 150 °C of that at 25 °C. As a result, the optimized phosphor shows an emission band peaked at 437 nm with a bandwidth of 40 nm and a high external photoluminescence quantum yield of 51.7%. Finally, a pc-WLED was fabricated by using NKMPO:Eu 2+ blue, Sr 2 SiO 4 :Eu 2+ green, CaAlSiN 3 :Eu 2+ red phosphors, and a 400 nm NUV diode chip. It shows a high color rendering index of R a $R_{\mathrm{a}}$  = 96.4 and a correlated color temperature of 4358 K. These results prove that NKMPO:Eu 2+ is a promising blue phosphor for full-spectrum WLED based on NUV diode chips.