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34 Handbook on the Physics and Chemistry of Rare Earths 1 0.5 0 ex. em 300 400 500 Wavelength (nm) 600 FIG. 29 Excitation and emission spectra of Ba2MgSi2O7:Eu2+. Redrawn from Toda, K., Kawakami, Y., Kousaka, S., Ito, Y., Komeno, A., Uematsu, K., Sato, M., 2006. New silicate phos- phors for a white LED. IEICE Trans. Electron. E89-C, 1406–1412; Komeno, A., Uematsu, K., Toda, K., Sato, M., 2006. VUV properties of Eu-doped alkaline earth magnesium silicate. J. Alloy Compd. 408–412, 871–874. On the other hand, the Ba2+/Eu2+ ions in the monoclinic structure have also eightfold oxygen coordination, but a Ba/EuO8 dodecahedron is much dis- torted with a noncentrosymmetric site as shown in Fig. 28. The average Ba/Eu–O bond distance in the monoclinic structure is 0.2854 nm, a rela- tively large value. Ba2MgSi2O7:Eu2+ has a broad absorption band ranging from UV (about 300 nm) to blue (about 430 nm) (Fig. 29). The phosphor is, therefore, able to efficiently absorb a wide range of wavelengths from UV to near-UV, violet, and blue, and it may be used as an excitation source for other phosphors. In addition, Ba2MgSi2O7:Eu2+ has broad emission spectrum in the green from 450 nm to 600 nm. Application of Ba2MgSi2O7:Eu2+ as a green phosphor for white LEDs can be expected, leading to a white light with a high CRI. The emission intensities at 100 and 150°C are 85% and 75% of that at 25°C, respectively. 4.1.2.4 Ba9Sc2Si6O24:Eu2+ Phosphor The crystal structure of Ba9Sc2Si6O24 was first determined by Wang et al. (1994) from single crystal XRD. Ba9Sc2Si6O24 has a rhombohedral structure with space group of R3, a1⁄40.9872 nm, and c1⁄42.1937 nm (Fig. 30). The structure is built of ScO6 octahedra and SiO4 tetrahedra corner sharing each other. Ba2+ ions are in three independent sites, BaO9 (M1), BaO10 (M2), and BaO12 (M3) polyhedra. The average Ba–O bond distances are 0.2899, Normalized intensity (a. u.)PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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