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Rare Earth-Doped Phosphors for White LEDs Chapter 278 23 they contain isolated SiO4. On the other hand, Ba3SiO5 has extremely low chemical stability against water, lower than that of Ba2SiO4. The luminescence properties of Eu2+-activated AE3SiO5 was investigated almost 40 years ago by Blasse et al. (1968a,b). The emission band of the phosphors shifts to a longer wavelength by increasing the atomic number of the alkaline-earth element. Actually, the maximum emission wavelengths of Eu2+-activated AE3SiO5 are 510, 545, and 590 nm for Ca, Sr, and Ba, respectively. Furthermore, Park et al. (2004, 2006) reported that the partial substitution of Sr2+ in the larger Ba2+ sites in Sr3SiO5:Eu2+ can red shift its emission. In addition, a white light LED based on a blue LED and the (Sr,Ba)3SiO5:Eu2+ phosphor have a higher CRI and a lower color temperature than those of conventional white LEDs combining YAG:Ce3+ with a blue LED. As shown in Fig. 15, the excitation spectra of (Sr1xBax)3SiO5:Eu2+ (x 1⁄4 0 and 0.25) consist of a broad band extending from 250 to 550 nm; the phosphors are consequently very suitable as color converters using any wave- length of excitation from a primary light source. As described earlier, the emission band of Sr3SiO5:Eu2+ shifts to a longer wavelength by substituting Ba for Sr sites, and then (Sr0.8Ba0.2)3SiO5:Eu2+ exhibits a broad yellow emis- sion with a peak at 580 nm, the tail of which covers the red region with high emission intensities. As to the red shift by Ba substitution, Park et al. (2004) pointed out the reason of it to be due to the lattice expansion along the c-axis and the lowered octahedral symmetry around Eu2+ sites. In addition, they also observed that the emission band was obviously red shifted by increasing the SiO2 content in the composition, being ascribed to a larger centroid shift aris- ing from an increase in covalency. However, on further increasing the Ba 1 0.5 0 200 ex. x=0 x=0.25 300 400 Wavelength (nm) em Excitation and emission spectra of (Sr1xBax)3SiO5:Eu2+ (x1⁄40 and 0.25). Redrawn from Shao, Q., Lin, H., Dong, Y., Fu, Y., Liang, C., He, J., Jiang, J., 2015. Thermostability and photo- stability of Sr3SiO5:Eu2+ phosphors for white LED applications. J. Solid State Chem. 225, 72–77. FIG. 15 500 600 700 Relative intensity (a.u.)PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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