Tuning of luminescence properties by controlling an aid-sintering additive and composition in Na(Ba/Sr/Ca)PO4:Eu2+ for white LEDs

Abstract

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.