Effect of lithium ion doping on cubic Gd_1.88Eu_0.12O₃: Enhancement of photoluminescence and thermal stability

Li⁺-doped cubic Gd_1.88Eu_0.12O₃ powders were synthesized at 1200 °C by the solid state reaction method. Under excitation of the charge transfer band of Eu³⁺ at 245 nm, the Gd_1.88Eu_0.12O₃ exhibited a dominant photoluminescence (PL) red emission peak at 611 nm, which is attributed to the electric dipole transition ⁵D₀→⁷F₂ of Eu³⁺. The dominant PL red emission peak intensity increased with Li⁺ doping and reached a maximum in the range from 12 to 16 mol%. This intensity was approximately 3 times higher than that of undoped Gd_1.88Eu_0.12O₃. Chemical composition analysis revealed that the Li⁺ species was completely evaporated from the samples by the heat treatment up to 1200 °C. However, during the heat treatment at 600-1000 °C, the doped Li⁺ could promote formation of a liquid phase, which enhanced the crystallite growth of cubic Gd_1.88Eu_0.12O₃ by Ostwald ripening. The thermal stability of the cubic Gd_1.88Eu_0.12O₃ was also improved with respect to the amount of Li⁺ doping. Even after annealing at 1300 °C (higher than the cubic/monoclinic phase transformation temperature of 1250 °C) for 72 h, X-ray diffraction pattern analysis indicated that the relative fraction of the cubic phase in the 12 mol% Li⁺-doped sample was as high as 90%. The relationship between the amount of Li⁺ doping and the asymmetric ratio evaluated for the PL emission spectrum and PL quantum efficiency of the cubic Gd_1.88Eu_0.12O₃ samples suggested that, in addition to enhanced crystallite growth, the formation of additional oxygen vacancies promoted by Li⁺ doping contributed to simultaneous enhancement in the PL red emission intensity and the thermal stability of cubic Gd_1.88Eu_0.12O₃ investigated in this study.