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Rare Earth-Doped Phosphors for White LEDs Chapter 278 31 1 ex. 0.5 0 300 CaSc2O4:Ce3+ Ca3Sc2Si3O12:Ce3+ em 400 500 600 700 Wavelength (nm) Excitation and emission spectra of CaSc2O4:Ce3+ and Ca3Sc2Si3O12:Ce3+. Redrawn from Shimomura, Y., Honma, T., Shigeiwa, M., Akai, T., Okamoto, K., Kijima, N., 2007. Photolumines- cence and crystal structure of green-emitting Ca3Sc2Si3O12:Ce3+ phosphor for white light emitting diodes. J. Electrochem. Soc. 154, J35–J38; Shimomura, Y., Kurushima, T., Kijima, N., 2007. Photo- luminescence and crystal structure of green-emitting phosphor CaSc2O4:Ce3+. J. Electrochem. Soc. 154, J234–J238. before, and however, the emission band of CaSc2O4:Ce3+ is located at a slightly longer wavelength (Henderson and Imbush, 1989; Tamaki and Murazaki, 2007). Such a contradiction is probably explained by considering that CaO8 in CaSc2O4 is somewhat more distorted than that of Ca3Sc2Si3O12. Namely, CaO8 in CaSc2O4 is close to regular antiprism, which usually results in a strong interaction between the 5d and 2p electrons of O2. The strong interaction con- tributes to enhance the crystal-field strength around Ce3+; consequently, causing the emission band shifting to longer wavelength. 4.1.2.3 Ba2MgSi2O7:Eu2+ Phosphors Recently, Eu2+-activated AE2MgSi2O7 (AE1⁄4Ca, Sr, and Ba) phosphors have been widely investigated as a phosphor for white LEDs and as long-lasting phosphors (Aitasalo et al., 2007; Gong et al., 2009; Kim et al., 2012; Tam et al., 2014; Wu et al., 2011). Here, we mainly discuss the luminescence behavior of Ba2MgSi2O7:Eu2+ that is also expected to be a suitable candidate as a green-emitting phosphor for white LEDs. The crystal structure of Ba2MgSi2O7 was first reported by Shimizu et al. on the basis of a single crystal X-ray investigation (Lin et al., 2003; Shimizu et al., 1995). The structure is a tetragonal akermanite-type structure with space group P421m. However, the powder X-ray diffraction (XRD) pat- tern of the Eu2+-activated Ba2MgSi2O7 phosphor was not identical to the reported tetragonal structure (Toda et al., 2006). Fig. 26 shows a comparison of the powder pattern with the pattern simulated from single crystal data. The FIG. 25 Normalized intensity (a. u.)PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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