Structure evolution and delayed quenching of the double perovskite NaLaMgWO₆:Eu³⁺ phosphor for white LEDs

Eu³⁺ ions activated NaLaMgWO₆ phosphors were successfully synthesized by an improved sol-gel method using citric acid and polyethylene glycol as complexing agents. Crystal structure and doping site were investigated by XRD, Rietveld refinement and Raman spectra. The phosphors had monoclinic double perovskite structure with space group C2/m, as well as layered ordering of A-site and rock-salt ordering of B-site. The blueshift of Raman T_2g(1) mode manifested Eu³⁺ ions had entered into A-site. Thereafter, luminescence properties, such as excitation and emission spectra, CIE coordinates, concentration quenching and thermal quenching were discussed. The quenching concentration for hypersensitive electric dipole transition of Eu³⁺ reached up to 50.0 mol%. The delayed concentration quenching was observed in NaLaMgWO₆: Eu³⁺ phosphor. The theoretical quenching concentration was obtained based on L. Ozawa's theory, and the quenching mechanism on Dexter's theory. Excellent thermal stability of this phosphor shows that it is a potential red phosphor for solid state lighting.