Enhanced Luminescence of Double Perovskite Na(La, Gd) MgWO6: Eu3+ Phosphor Based on A-Site-Induced Energy Transfer

Gd3+ ions substituted Na(La0.98- xGdxEu0.02)(2)MgWO6 phosphors with ordered double-perovskite structure were successfully synthesized by an improved citrate-EDTA complexing method. The structure's, changes and the resulting luminescence variation were systematically investigated by X-ray diffraction with Rietveld structure refinement, Raman spectra, Fourier transform infrared spectra, excitation and emission spectra. The decreased symmetry and lattice parameters within the same space group C2/m were observed from the Gd3+ substituted powders. The substitution of Gd3+ made the T-2g(1) Raman mode to broaden and blueshift due to a decreased lattice volume, an enhanced covalent bonding effect, and "Lanthanide contraction" phenomenon. The excitation intensity ratio of CTB/4f-4f reversed when x >= 0.7. The intensity of D-5(0)-F-7(2) monotonically increased under the excitations of both near UV and blue light, being enhanced nine and four times, respectively, and they becaming almost equal when x = 0.98. The relative intensity ratio of red/orange emission gradually increased and the CIE chromaticity coordinates gradually approached the standard red light. The substitution of Gd3+ not only enhanced the luminescence intensity and acceptance for different excitation chips, but also greatly improved the red light quality. "A site induced energy transfer" in double perovskite that results in the luminescence enhancement of Eu3+, was achieved by selecting the substitution element with the small radius but big electronegativity. The radiation transition of electric dipole of Eu3+ was greatly enhanced due to the low crystal symmetry, increased covalence of Eu3+-O2-, and especially the shorted Eu-O bond length.