Structure and luminescent properties of Dy3+ activated NaLa9(SiO4)6O2 yellow-emitting phosphors for application in white LEDs

A series of Dy3+-doped sodium lanthanum orthosilicate oxyapatites, NaLa9−xDyx(SiO4)6O2 (NLSO:xDy, x = 0–0.3), were synthesized by the solid-state reaction method at 1100 °C, with the aim of developing yellow emitting phosphors for applications in w-LEDs. Their crystal structure, morphology and particle size distribution, electronic structure, Raman spectra, concentration- and temperature-dependent luminescent properties were investigated for the first time. The crystal structure was refined by the Rietveld method. Undoped NLSO is hexagonal (space group P63/m – C26h) with lattice constants a = b = 9.6917(3) Å and c = 7.1836(4) Å. It is shown that Dy3+ ions substitute for the La3+ ones in two types of sites with C3 and Cs point symmetries. The first-principle calculations for undoped NLSO revealed an indirect bandgap of 5.06 eV. The NLSO:0.2Dy phosphor exhibited intense yellow emission with CIE 1931 chromaticity coordinates of (0.469, 0.495) and a correlated color temperature of 3150 K owing to the hypersensitive electric-dipole transition 4F9/2→6H13/2 (571 nm) dominating in the luminescence spectrum. Excellent thermal stability was found for this apatite phosphor (the activation energy is 0.23 ± 0.02 eV).