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56 Handbook on the Physics and Chemistry of Rare Earths 100 FIG. 52 80 60 40 20 0 50 100 150 200 Temperature (°C) Temperature dependence of the emission intensity of CaAlSiN3:Eu2+ (CASN). Redrawn from Piao, X., Machida, K., Horikawa, T., Hanzawa, H., Shimomura, Y., Kijima, N. 2007. Prepara- tion of CaAlSiN3:Eu2+ phosphors by the self-propagating high-temperature synthesis and their luminescent properties. Chem. Mater. 19, 4592–4599. up to 150°C, the emission intensity of CASN decreases to 89% of that at room temperature (25°C) (Xie et al., 2006a,b, 2007, 2010). At the same time, fluctu- ation of the emission band position is very small, actually negligible. Besides blue light absorption, CASN has a strong optical absorption in green region, leading to a reduction in the CRI of white LEDs combining a blue LED with green and red phosphors. In addition, the emission band of CASN located in the deep-red region (655 nm) is also responsible for the reduction of the luminescence efficiency of white LEDs. To solve these problems, several approaches, such as the formation of solid solutions of CaAlSiN3–SrAlSiN3 or CaAlSiN3–Si2N2O and the use of alloy precursors in the synthesis, have been reported (Mikami et al., 2008, 2009a,b; Piao et al., 2007a; Watanabe and Kijima, 2009; Watanabe et al., 2008a,b). Among them, the formation of CaAlSiN3–SrAlSiN3 solid solution is the most promising approach to improve the luminescence of CASN without loss of quantum yield. Since the ionic radius of Sr2+ (0.118 nm for sixfold coordination) is larger than that of Ca2+ (0.100 nm) (Shannon, 1976), par- tial substitution of Sr2+ for Ca2+ results in the reduction of the crystal-field strength around Eu2+, contributing to blue shifts in excitation and emission bands. The emission band of SrxCa1xAlSiN3:Eu2+ effectively shifts to a shorter wavelength with increasing Sr2+ content (Fig. 53). Particularly, Sr0.8Ca0.2AlSiN3:Eu2+ shows a highly pure red emission with peaking at 630 nm under excitation at 450 nm. Furthermore, the decrease in the emis- sion intensity by thermal quenching is also successfully minimized to only 10% at 150°C. Relative intensity (%)PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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