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58 Handbook on the Physics and Chemistry of Rare Earths TABLE 7 Structure Data of AE2Si5N8 (AE5Ca, Sr, and Ba) Ca2Si5N8 (Schlieper and Schnick, 1995) Crystal System Monoclinic Orthorhombic Orthorhombic b (nm) 0.5610(1) 0.6822(1) 0.6959(1) Sr2Si5N8 (Schlieper et al., 1995) Ba2Si5N8 (Schlieper et al., 1995) Space group Cc Pmn21 Pmn21 a (nm) 1.4352(3) 0.5710(1) 0.5783(1) c (nm) 0.9689(2) 0.9341(2) 0.9391(2) a (°) 90.0 90.0 90.0 b (°) 112.06(3) 90.0 90.0 g (°) 90.0 90.0 90.0 V (nm3) 0.723 0.36387 0.37793 Z422 significantly larger than that for Sr2Si5N8. The formation of an all- proportional solid solution between AE2Si5N8 (AE1⁄4Sr and Ba) and Eu2Si5N8 is possible, indicating that Eu2Si5N8 is also isostructural. Emission spectra of AE2xEuxSi5N8 (AE1⁄4Ca, Sr, and Ba; x1⁄40.02) are shown in Fig. 55, and excitation and emission spectra of Sr0.98Eu0.02Si5N8 are shown in Fig. 56. The excitation spectrum consists of a broad band rang- ing from 220 to 600 nm. Although the peak positions in excitation spectra of Ca2Si5N8:Eu2+ and Ba2Si5N8:Eu2+ are different from Sr2Si5N8:Eu2+, both phosphors have a strong broad optical absorption in the near-UV and blue region. A common feature for the three phosphors is the broad red emission band peaking at around 605, 610, and 574 nm for Ca, Sr, and Ba, respec- tively (Li et al., 2006a). As the Eu2+ content increases from x1⁄40.02 to 2.00, the emission color of the phosphors vary from yellow to deep-red (from 590 to 680 nm) in the case of Sr2Si5N8:Eu2+ and from green to deep-red (from 560 to 680 nm) in the case of Ca2Si5N8:Eu2+. Under excita- tion at 465 nm, the emission intensity of the phosphors increases in the order Ca2Si5N8:Eu2+ < Ba2Si5N8:Eu2+ < Sr2Si5N8:Eu2+. When Sr2Si5N8:Eu2+ is excited at 450 nm, the external quantum efficiency is $64%. As shown in Fig. 57, the Sr2Si5N8:Eu2+ phosphor exhibits a thermal quenching behavior similar to that of CASN. The emission intensity of the phosphor measured at 150°C is 86–90% of that at 25°C.PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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