Tuning of luminescence properties by controlling an aid-sintering additive and composition in Na(Ba/Sr/Ca)PO4:Eu2+ for white LEDs
A series of Na(Ba/Sr/Ca)PO4:Eu2+ phosphors were prepared via a high-temperature solid-state reaction method. When the phase of NaCaPO4:Eu2+ was pure, the luminescence of Eu2+ was enhanced by doping the sintering-aid additive NaCl (t), and it showed a maxima at t = 0.03. For NaCaPO4:Eu2+ with 0.03NaCl, the XRD patterns, and emission and decay spectra demonstrated that Eu2+ ions occupied two different Ca sites with different coordination (i.e., eight and seven coordination). Therefore, two green emission bands at 510 and 542 nm were observed, and the emission band ranging from 480 to 510 nm had a weaker intensity. To obtain a cyan-broad emission, Ba2+ or Sr2+ were introduced into NaCaPO4:0.01Eu2+. The substitution of small Ca2+ ions by large Ba2+ or Sr2+ ions induced a decreased crystal field splitting of Eu2+ ions, which resulted in various full width at half maximum and a blue shift. The colors varied from green (0.1996, 0.4380) to blue (0.1578, 0.0978) under the same excitation. Overall, the phosphor has promising applications for use in white LEDs.