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20 Handbook on the Physics and Chemistry of Rare Earths MI site (M = Ba) AE (MI) SiO4 AE (MII) b c a Crystal structure of AE2SiO4 (AE1⁄4Ba, Sr, and Ca). The unit of bond distance is nm. Pictures were drawn based on the structure data obtained from the database (ICSD #6246). an orthorhombic a0-type structure. The crystal structure of (Ba,Sr)2SiO4 is illustrated in Fig. 11. There are isolated SiO4 tetrahedra. For Sr2SiO4, the average bond length of Sr–O is 0.2855 nm for Sr(I) and 0.2702 nm for Sr(II). The photoluminescence properties of Eu2+-activated (Ba,Sr)2SiO4 phos- phors were first reported by Barry (1968) and have recently begun to receive attention again as conversion phosphors excitable by blue LEDs. The excita- tion and emission bands of (Ba,Sr)2SiO4:Eu2+ can be tuned by controlling the Ba/Sr ratio (Barry, 1968). The Ba-rich (Ba,Sr)2SiO4:Eu2+ phosphors with Ba:(Sr/Eu) ratio of about 1:1 exhibit a green emission, also available as a conversion phosphor for white LEDs, while the Sr-rich (Ba,Sr)2SiO4:Eu2+ phosphors exhibit a yellow emission. Here, we mainly discuss the lumines- cence properties of the yellow-emitting Sr-rich (Ba,Sr)2SiO4:Eu2+ phosphors. The excitation spectrum shows a broad absorption band ranging from 220 to 500 nm caused by both the CT from oxide ions to Eu2+ ions and the 4f7!4f65d1 allowed transition of Eu2+ (Fig. 12). The absorption band well matches with a blue light emitted from an InGaN LED chip. The emission spectrum exhibits a broad yellow emission predominating at 575 nm. In comparison with YAG:Ce3+, Sr-rich (Ba,Sr)2SiO4:Eu2+ show a higher emission intensity in the red spectral region and also have a higher CRI. FIG. 11 0.2640(7) 0.29153(8) 0.2904(5) 0.2904(5) 0.3083(7) 0.3174(5) 0.3174(5) 0.29153(8) 0.3060(5) 0.3060(5) MII site (M = Ba) 0.2755(5) 0.3113(3) 0.2755(5) 0.2759(6) 0.3113(3) 0.2846(7) 0.2686(5) 0.2699(7) 0.2686(5)PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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