Crystal phase control of red-emitting Li4MgWO6:Mn4+ phosphors using LiF for high luminescence efficiency

Abstract

Phosphor-converted white light-emitting diodes (pc-WLEDs) are widely used owing to their high efficiency, long life, high reliability, low cost, and environmental friendliness. However, their color rendering remains limited because of the lack of red emissions. In this study, we synthesized a rare-earth-free, Mn⁴⁺-activated Li₄MgWO₆ red-emitting phosphor through a solid-state reaction method at only 850 °C in 1 h, by partially substituting Li₂CO₃ with LiF as a Li source to enhance the phase purity and crystallinity. X-ray diffraction and luminescence measurements revealed that the monoclinic Li₄MgWO₆:Mn⁴⁺ phosphor synthesized without LiF had a low crystallinity and weak luminescence, whereas the addition of LiF promoted the formation of a highly crystalline orthorhombic phase with strong red emission. The optimized phosphor Li₄MgW_0.998O₆:0.002Mn⁴⁺ with the orthorhombic structure exhibited a broad excitation band in the UV–blue region and strong red emission centered at 665 nm under 319 nm excitation, corresponding to the ²E_g→⁴A_2g transition of Mn⁴⁺, with an internal quantum efficiency of 64.5 %. This LiF-assisted synthesis strategy for Li₄MgW_0.998O₆ can be extended to other Li-based rock-salt-type materials, improving the luminescence performance of red phosphors for pc-WLEDs.