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36 Handbook on the Physics and Chemistry of Rare Earths 1.2 1 0.8 0.6 0.4 0.2 0 ex. em 300 400 500 Wavelength (nm) 600 Photoluminescence excitation and emission spectra of the Ba9Sc2Si6O24:Eu2+. Redrawn from Nakano, T., Kawakami, Y., Uematsu, K., Ishigaki, T., Toda, K., Sato, M., 2009. Novel Ba–Sc–Si–oxide and oxynitride phosphors for white LED. J. Lumin. 129, 1654–1657. for white LEDs by Kakihana’s group (Sato et al., 2014; Tezuka et al., 2013). Ca2SiO4:Eu2+ is well known as a green-emitting phosphor excitable with near-UV light (Choi and Hong, 2009; Kim et al., 2005a,b; Luo et al., 2012). Ca2SiO4 has five different morphologies, g-phase, b-phase, a0L-phase, a0H- phase, and a-phase (Smith et al., 1965; Yamnova et al., 2011), as summarized in Table 2. Among them, the g-Ca2SiO4 phase with space group Pcmn is gen- erally stable at room temperature, and the metastable b-phase can be stabi- lized at room temperature with addition of various dopants (Kim and Hong, 2004; Lai et al., 1992). Therefore, almost all of the reported Eu2+-activated Ca2SiO4 phosphors have a stabilized monoclinic b-phase structure with space group P21/n. The phosphors show a broad green emission band with a peak at around 502 nm (Choi and Hong, 2009; Luo et al., 2012). In the meantime, Kakihana’s group recently reported that the emission band of Eu2+-activated Ca2SiO4 shifts to a longer wavelength upon increasing the Eu2+ content (Sato et al., 2014). Ca2xEuxSiO4 undergoes a phase transformation from b-phase to a0L-phase when the Eu2+ content is in the range between x1⁄40.1 and 0.2. Fig. 32 shows the excitation and emission spectra of the a0L-phase phosphor with x1⁄40.2 and 0.8. The phosphor with x1⁄40.2 exhibits two broad green–yellow emission bands with maxima at 560 and 620 nm under excita- tion at 450 nm. Furthermore, the emission bands shift to longer wavelengths with increasing Eu2+ content. The phosphor with x1⁄40.8 has a broad red emis- sion band with a peak at 650 nm, which is similar to that of CaAlSiN3:Eu2+. The emission band shift from green–yellow to deep-red is dependent on which sites the emission ion prefers among several acceptable sites. It is well known that alkaline-earth orthosilicate phosphors AE2SiO4 have two independent FIG. 31 Normalized intensity (a. u.)PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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