Luminescence properties of novel emission-tunable NaSr(4−x−y)Bax(BO3)3:yEu2+ phosphors for white light emitting diodes

Abstract

A series of emission-tunable NaSr_(4−x−y)Ba_x(BO₃)₃:yEu²⁺ phosphors have been prepared by a conventional solid-state reaction method. The structures of NaSr_(4−x−y)Ba_x(BO₃)₃:yEu²⁺ have been investigated by Rietveld refinement of the X-ray diffraction (XRD) patterns. The results indicated that the as-prepared samples showed the same crystal structure of NaSr₄(BO₃)₃ with a cubic unit cell and space group of Iad. With the increase of Ba²⁺ concentration, the Sr²⁺ sites were replaced by Ba²⁺ completely and the lattice parameter of the unit cell increased from a = b = c = 15.0710 Å to 15.7266 Å. Both emission spectra and decay curves of NaSr_3.98(BO₃)₃:0.02Eu²⁺ and NaBa_3.98(BO₃)₃:0.02Eu²⁺ showed the existence of two different Eu²⁺ emission centers named Eu1 and Eu2. Eu2 was six-coordinated and Eu1 was eight-coordinated of oxygen. With the increase of Eu²⁺ concentration in the NaSr_3−yBa(BO₃)₃:yEu²⁺ sample, the emission intensity increased and reached a maximum at y = 0.02. Then the concentration quenching phenomenon emerged due to the electric dipole–dipole interaction. Upon the cation substitutions (Sr²⁺ for Ba²⁺) in the NaSr_(4−x−y)Ba_x(BO₃)₃:yEu²⁺ host, the emission peaks of Eu²⁺ blue-shifted from 609 nm to 544 nm and the thermal stability decreased, which was ascribed to the change of the covalency and the crystal field strength that the 5d orbital of the Eu²⁺ ion experiences. The CIE chromaticity coordinates of the obtained phosphors can be continuously tuned from orange-red (0.4795, 0.4070) to yellow-green (0.3432, 0.4665) by adjusting the Ba²⁺ concentration. The results demonstrate that the emission-tunable NaSr_(4−x−y)Ba_x(BO₃)₃:yEu²⁺ phosphors have a potential application for white light emitting diodes (w-LEDs).