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28 Handbook on the Physics and Chemistry of Rare Earths covering the region from near-UV to visible (400–500 nm). The emission spectrum exhibits a broad reddish–yellow band ranging from 480 to 750 nm with a peak at 583 nm under excitation at 437 nm. The emission peak wave- length of the phosphor is at longer wavelength compared to the commercial yellow-emitting YAG:Ce3+ phosphor (555 nm). 4.1.2 Green-Emitting Phosphors 4.1.2.1 Ca3Sc2Si3O12:Ce3+ Phosphors Ca3Sc2Si3O12:Ce3+ (CSS:Ce3+) phosphors, reported in 2007 by Shimomura, are of the garnet-type phosphors (Shimomura et al., 2007a). Ca2+, Sc3+, and Si4+ ions occupy dodecahedral, octahedral, and tetrahedral sites, respectively. The lattice parameter a 1⁄4 1.225 nm is larger than that of YAG (1.2013 nm). In addition, Ce3+ ions preferably occupy the Ca2+ sites because the ionic radius of Ce3+ (0.128 nm for eightfold coordination and 0.115 nm for sixfold coordi- nation; Shannon, 1976) is similar to that of Ca2+ (0.126 nm for eightfold coor- dination; Shannon, 1976), and much larger than that of Sc3+ (0.089 nm for sixfold coordination; Shannon, 1976). It is reasonable to consider that charge compensation is achieved by the formation of Ca vacancies because the formal valences of Ce and Ca ions are 3+ and 2+, respectively. The excitation spectrum of CSS:Ce3+ consists of a strong broad absorption band covering the range of 380–500 nm with a peak at 455 nm (Fig. 22), which is very suitable for the color converter of a blue-LED chip as primary light 1 0.5 ex. em 0 300 400 500 600 700 Wavelength (nm) Excitation and emission spectra of CSS:Ce3+. Redrawn from Shimomura, Y., Honma, T., Shigeiwa, M., Akai, T., Okamoto, K., Kijima, N., 2007. Photoluminescence and crystal structure of green-emitting Ca3Sc2Si3O12:Ce3+ phosphor for white light emitting diodes. J. Electrochem. Soc. 154, J35–J38. FIG. 22 Normalized intensity (a.u.)PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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