Massive red shift of Ce3+ in Y3Al5O12 incorporating super-high content of Ce. Issue 21 (27th March 2020)

• Record Type: Journal Article
• Authors: Nakamura, Hitomi; Shinozaki, Kenji; Okumura, Toyoki; Nomura, Katsuhiro; Akai, Tomoko
• Is Part Of: RSC advances. Volume 10:Issue 21(2020)
• Page Start: 12535
• Page End: 12546
• Publication Date: 2020-03-27
• Subjects: Chemistry -- Periodicals; 540.5
• DOI: 10.1039/d0ra01381a
• Languages: English
• ISSNs: 2046-2069

Abstract

Abstract : In light emitting diodes, Y3 Al5 O12 :Ce (YAG:Ce) is used as a yellow phosphor in combination with blue LEDs but lacks a red component in emission. Abstract : In light emitting diodes, Y3 Al5 O12 :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 Y3 Al5 O12 (YAG) crystal incorporating a high content of Ce, (Y1− x Ce x )3 Al5 O12 (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. CeLIII X-ray absorption edge structure revealed that Ce existed as Ce 4+ in the precursor and Ce 3+ 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 Y 3+ 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.