Synthesis and luminescence properties of efficient red phosphors SrAl4O7:Mn4+,R+ (R+=Li+, Na+, and K+) for white LEDs

A series of SrAl[4−(4x)/3]O7:xMn4+ (0≤x≤1.4 mol%) and Sr(1−0.06/2)Al[4−(4×0.008)/3]O7:0.008Mn4+,0.06R+ (R+=Li+, Na+, and K+) phosphors are synthesized by a conventional solid-state reaction method in air. The PLE bands peaking at~325, 395, and 470 nm and the strongest PL band peaking at~660 nm are assigned to the 4A2→4T1 transition of Mn4+ ion and O2−–Mn4+ change transfer band, 4A2→2T1, 4A2→4T2, and 2E→4A2 transitions, respectively. The optimum Mn4+ doping concentration is about 0.8 mol%. Li+, Na+, and K+ ions used as charge compensator can improve observably the PL intensity of SrAl4O7:Mn4+ phosphor with excitation 325 nm. The fluorescence lifetimes of SrAl[4−(4×0.008)/3]O7:0.008Mn4+ and Sr[1−0.06/2]Al[4−(4×0.008)/3]O7:0.008Mn4+,0.06 R+ (R+=Li+, Na+, and K+) are 0.958, 0.97, 1.113 and 0.965 ms, respectively. The luminous mechanism is explained by schematic energy level diagram of Mn4+ ion. These experiment results indicate that SrAl4O7:Mn4+,R+ (R+=Li+, Na+, and K+) phosphors can be used as red emitting phosphor candidate for white LEDs based on a UV (~325 nm) and near UV (~395 nm) bands of LEDs chip.