Enhanced luminescence of double perovskite Na(La, Gd)MgWO₆: Eu³⁺ phosphor based on a-site-induced energy transfer

Gd³⁺ ions substituted Na(La_0.98-xGd_xEu_0.02)₂MgWO₆ 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 ⁵D₀-⁷F₂ 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⁺-O^2-, and especially the shorted Eu-O bond length.