Tunable emission color and mixed valence state via the modified activator site in the AlN-doped Sr3SiO5:Eu phosphor

Abstract

Commercially white LEDs have cool white light and a low color rendering index (CRI) due to the absence of red light emission. Considering the respective characteristic luminescence properties of Eu²⁺ and Eu³⁺ ions, the combination of the two activators into a single host compound shows promisingly technical feasibility to realize warm w-LEDs with a high CRI but it is still a challenge in practice. Here, we confirmed the mixed europium valence states in Sr₃Si_1−xAl_xO_5−2xN_x:Eu phosphors, synthesized by a solid-state reaction method under a H₂/N₂ atmosphere. The crystal structure was characterized by X-ray diffraction and found to crystallize in the Sr₃SiO₅ phase. The samples show broad excitation bands (250–400 nm) and tunable emission colors from yellow to red, and the CIE chromaticity coordinates of Sr_2.97Si_1−xAl_xO_5−2xN_x:0.03Eu (x = 0, 0.03, 0.2) were (0.518, 0.474), (0.607, 0.385) and (0.630, 0.367), respectively. The modification of activator sites caused by Al³⁺–N³⁻ replacing Si⁴⁺–O²⁻ explained the coexistence of Eu²⁺/Eu³⁺ ions, which was demonstrated by Rietveld refinement, photoluminescence analysis and fluorescence lifetime studies. Furthermore, the obtained phosphors also possessed good thermal stabilities. These results suggest that the Sr₃Si_1−xAl_xO_5−2xN_x:Eu phosphors show potential applications in w-LEDs.