Second-order John-Teller distortion in the thermally stable Li(La, Gd)MgWO₆:Eu³⁺ phosphor with high quantum efficiency

Thermally stable Li(La, Gd)MgWO₆:Eu³⁺ phosphors with high quantum efficiency were obtained by the sol-gel method. The symmetry of this double perovskite was reduced by doping Gd³⁺ ions in order to enhance the luminescence properties. In addition, phase structure and luminescence properties were characterized. A second-order John-Teller (SOJT) distortion of WO₆ octahedron existed in this double perovskite and was investigated in detail for the first time. The WO₆ octahedron of LiLa_0.95-xGd_xEu_0.05MgWO₆ distorted gradually and the SOJT distortion started to disappear while x ≥ 0.7. Intensities of ⁵D₀-⁷F₂ transition were greatly enhanced and reached the maximum at x = 0.5 under the excitation of 465 nm. The LiLa_0.45Gd_0.5Eu_0.05MgWO₆ phosphor showed excellent thermal stability and the thermal quenching mechanism was discussed by the configurational coordinate diagram. The optimized LiGd_0.5Eu_0.5MgWO₆ showed high quantum efficiency, which is much higher than commercial Y₂O₂S:Eu³⁺ and Y₂O₃:Eu³⁺ phosphors. The synthesized phosphors emit intense red light under the near ultraviolet and have great potential in light emitting diode.