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FIG. 71 Rare Earth-Doped Phosphors for White LEDs Chapter 0.2842 278 73 Sr/Ba F SiO4 0.2842 F 0.2786 0.2842 0.2842 0.2460 cF F 0.2519 Sr2 a b 0.2697 0.2519 0.2697 Crystal structures of (Sr,Ba)3AlO4F. The unit of bond distance is nm. Pictures were drawn based on the structure data obtained from the database (ICSD #50736). average Sr–F and Sr–O bond distances in the SrO6F2 polyhedra are 0.2572 and 0.2638 nm, respectively. The average Ba–F and Ba–O bond distances in the BaO8F2 polyhedra are 0.2801 and 0.2891 nm, respectively (Im et al., 2010). The dopant Ce3+ ions preferably occupy the Sr2+ site because the ionic radius of Eu2+ (0.125 and 0.135 nm for 8 and 10 coordination) is similar to that of Sr2+ (0.126 nm for 8 coordination), while the ionic radius of Ba2+ (0.152 nm for 10 coordination) is much larger than (Shannon, 1976). The excitation spectrum of (Sr,Ba)3AlO4F:Ce3+ includes two bands, corresponding to the 4f!5d transitions of Ce3+, with the strongest absorption ranging between 380 and 450 nm (Im et al., 2010) (Fig. 72). Under a 400 nm excitation, the emission spectrum exhibits a broad green emission band with maximum at 502 nm. The composition of Sr1.975BaCe0.025AlO4F gives the highest emission intensity, 1.5 times higher than that of a YAG:Ce3+ (P46) phosphor. Furthermore, the quantum efficiency of the phosphor is approximately 95% at room temperature under excitation at 400 nm. How- ever, the emission intensity of (Sr,Ba)3AlO4F:Ce3+ decreases rapidly with increasing the temperature. At 200°C, the relative emission intensity is down to 50% of that measured at room temperature (25°C). The luminous efficacy of a white LED fabricated by coating a (Sr,Ba)3AlO4F:Ce3+ phosphor on a 405 nm InGaN chip is 10–15 lm/W and the CRI (Ra) is approximately 62. Denault et al. (2012) have also reported that the luminescence efficiencies of (Sr,Ba)3AlO4F:Ce3+ are effectively enhanced by doping a small amount of SiO2 component into the (Sr,Ba)3AlO4F:Ce3+ lattice, leading to a com- pound with formula Sr2Ba(AlO4F)1x(SiO5)x:Ce3+. 0.2460 0.2842 0.2786 F 0.2842 0.2697 Sr1 0.2842 0.2697PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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