Dual-channel enhanced luminescence of double perovskite NaGdMgWO₆:Eu³⁺ phosphor based on alternative excitation and delayed quenching

Series of Eu³⁺ substituted Na(Gd_1-xEu_x)MgWO₆ phosphors were successfully synthesized by a EDTA-citric acid complexing method. The crystal structure, doping site, morphology, and luminescent properties under different Eu³⁺ concentrations were investigated by XRD, Rietveld refinement, Raman spectra, FTIR, SEM and fluorescence spectra as well as Judd-Ofelt analysis and quenching theory. The phosphors had monoclinic double perovskite structure with space group C2/m, rock-salt ordering of B-site (Mg/W) and layered ordering of A-site (Na/Gd/Eu). The redshift of Raman T_2g(1) mode proved Eu³⁺ ions had uniformly substituted into A-site. The rare alternative excitation appeared in phosphors that the charge transfer band (CTB) altered from O-W into O-Eu and the strongest excitation altered from CTB into f-f transition of Eu³⁺. The quenching concentration for hypersensitive ⁵D₀-⁷F₂ transition of Eu³⁺ reached up to 50.0 mol%, while both the intensity ratio of ⁵D₀-⁷F₂/⁵D₀-⁷F₁ and Judd-Ofelt parameter Ω₂ reached the maximum. These two channels of alternative excitation and delayed concentration quenching dually enhanced the luminescence of NaGdMgWO₆:Eu³⁺ phosphor.