High luminescent thermal stability and water resistance of K2SiF6:Mn4+@CaF2 red emitting phosphor

K2SiF6:Mn4+ (KSF:Mn4+), as an efficient red-emitting phosphor, has a promising application in WLEDs (white light-emitting diodes). However, poor moisture resistance performance still hinders its deeper commercialization. Here, KSF:Mn4+@ CaF2 with high water resistance and luminescent thermal stability has been prepared though H2O2-free hydrothermal method and surface coating process. Both KSF:Mn4+ and KSF:Mn4+@CaF2 all have high luminescent thermal stability, due to negative thermal quenching (NTQ) effect. Mechanism of the NTQ has been discussed and suggested as thermal-light energy conversion mechanism. Compared with KSF:Mn4+, water resistance of KSF:Mn4+@CaF2 is greatly improved by coating of CaF2, because the outer shell of CaF2 can effectively prevent the [MnF6]2- group on the surface of the phosphor from being hydrolyzed into MnO2. The results of water resistance test shows that after immersing in water for 360 min (6 h), luminescent intensity of the uncoated product drops to 41.68% of the initial one, while that of the coated product remains to have 88.24% of its initial one. Warm white light with good luminescent performances (CCT = 3956 K and Ra = 89.3) is got from prototype WLEDs assembled by using the optimal coated sample. The results suggest that the optimal coated sample has potential application in blue-based warm WLEDs.