A novel red phosphor Li_xNa_1-xEu(WO₄)₂ solid solution: Influences of Li/Na ratio on the microstructures and luminescence properties

A series of double tungstate solid-solution red phosphors Li_xNa_1-xEu(WO₄)₂ (0 ≤x ≤ 1) have been successfully fabricated via the traditional solid-state reaction method, and the influences of Li/Na ratio on the microstructures and luminescence properties of the as-fabricated Li_xNa_1-xEu(WO₄)₂ crystals were investigated in detail. The XRD and FT-IR results show that all the as-fabricated microcrystals are tetragonal symmetry with scheelite structure, and the doping concentration of Li⁺ has great influence on the lattice parameters of the finally obtained Li_xNa_1-xEu(WO₄)₂ crystals. The SEM results indicate that all samples are well crystallized, exhibited irregular spherical particles with various mean grain size. Moreover, the mean grain size of the final Li_xNa_1-xEu(WO₄)₂ solid solution crystals increases with increasing concentration of Li⁺ ions besides LiEu(WO₄)₂ and NaEu(WO₄)₂. The luminescence spectra reveal that all the Li_xNa_1-xEu(WO₄)₂ phosphors can exhibit the two characteristic emissions located at 615 nm and 592 nm under 393 nm UV excitation. Moreover, the observed emission intensities of the emission peak at 615 nm are obviously fluctuant with x, whereas the intensities of the emission peak located at 592 nm have no obvious variation. All the obtained Li_xNa_1-xEu(WO₄)₂ phosphors can emit warm pure red emissions with the correlated color temperature (CCT) from 2100 to 2300 K. Especially, for the as-fabricated Li_.1Na_.9Eu(WO₄)₂ phosphor, the coordinate (x = 0.6508, y = 0.3474) are much closer to the International Association of Lighting NTSC trichromatic coordinates specified red coordinate (x = 0.67, y = 0.33) than that of Y₂O₂S: Eu³⁺ (x = 0.63, y = 0.35), showing that is a promising warm red phosphor with excellent luminescence properties.