Massive red shift of Ce³⁺ in Y₃Al₅O₁₂ incorporating super-high content of Ce

In light emitting diodes, Y₃Al₅O₁₂:Ce (YAG:Ce) is used as a yellow phosphor in combination with blue LEDs but lacks a red component in emission. Therefore, considerable efforts have been directed toward shifting the emission of YAG:Ce to longer wavelengths. In this study, a Y₃Al₅O₁₂ (YAG) crystal incorporating a high content of Ce, (Y_1- Ce )₃Al₅O₁₂ (0.006 ≦ x ≦ 0.21), was successfully prepared by a polymerized complex method in which low-temperature annealing (650-750 °C) was employed prior to sintering at 1080 °C. X-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis indicated that the obtained sample was a single phase YAG crystal with x ≤ 0.21. Interestingly, orange-red emission was observed with x ≥ 0.07 with UV-blue light irradiation. With excitation at 450 nm, the emission peak increases from 538 nm (x = 0.006) to 606 nm (x = 0.21). This massive red shift in the high-x region was not observed without the 1^st step of low-temperature annealing, which implied that low-temperature annealing was essential for incorporating a high concentration of Ce. The precursor formed by low-temperature annealing was amorphous at x = 0.04, whereas CeO₂ nanocrystals were formed in the amorphous material with x ≥ 0.11, based on the XRD and TEM results. CeL_III X-ray absorption edge structure revealed that Ce existed as Ce⁴⁺ in the precursor and Ce³⁺ in the obtained crystal. It was speculated that CeO₂ was formed at low temperature, releasing oxygen, with sintering at 1080 °C, leading to the incorporation of Y³⁺ in the Ce-O framework. The lattice constant increased significantly from 12.024 Å to 12.105 Å with increasing x, but the crystal field splitting did not increase and was constant from x = 0.06 to x = 0.21. Hence, the massive red shift in emission was not explained by the large crystal field splitting, but instead by the Stokes shift.