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Rare Earth-Doped Phosphors for White LEDs Chapter 278 51 TABLE 4 Photoluminescence Data of AESi2O2N2:Eu2+ (AE5Ca, Sr, Ba) Phosphors (Li, et al., 2005) Excitation Emission Absorption Stokes QE AE Band (nm) Band (nm) Edgea (nm) Shift (cm21) (ex.)b (%) Ca 259, 341, 560 $280 $5100 72 395, 436 Sr 260, 341, 530–570 $270 $3900–5200 69 387, 440 Ba 264, 327, 499 $240 $1700 41 406, 480 aAESi2O2N2 host material. bExternal quantum efficiency. yellow emission band with a maximum at 560 nm (Bachmann et al., 2009b). Under excitation at 450 nm, the external quantum efficiencies of these phos- phors are 72%, 69%, and 41% for CaSi2O2N2:Eu2+, SrSi2O2N2:Eu2+, and BaSi2O2N2:Eu2+, respectively (Gu et al., 2010). The detailed photolumines- cence data of AESi2O2N2:Eu2+ (AE 1⁄4 Ca, Sr, and Ba) are summarized in Table 4 (Li et al., 2015a,b). In the case of SrSi2O2N2:Eu2+, the emission inten- sity measured at 150°C is !80% of that at room temperature. The excellent luminescence efficiencies of AESi2O2N2:Eu2+ (AE 1⁄4 Ca, Sr, and Ba) make them very suitable for use in near-UV or blue-LED-based white LEDs as a conversion phosphor. 4.2.3.3 Y4Si2O7N2:Ce3+ Phosphors The host phase of Y4Si2O7N2:Ce3+ phosphors is a compound existing in the Y–Si–O–N quaternary system. Y4Si2O7N2 has a crystal structure different from N-melilite type Y2Si3O3N4 (described in Section 4.2.2) as shown in Fig. 49. Y4Si2O7N2 crystallize in a monoclinic structure with space group P21/c (so-called J-phase) (MacKenzie et al., 1996). In Table 5 are summarized the crystallographic data of N-melilite Y2Si3O3N4 and J-phase Y4Si2O7N2. In Y4Si2O7N2, two Si(O/N)4 tetrahedra are coupled by corner sharing (nitrogen atom) to form Si2O5N2 di-tetrahedral units, running along the a-axis. The Y3+ ions have four independent sites with different crystallographic environ- ments, giving one kind of Y(O/N)6 and three kinds of Y(O/N)7 motifs (MacKenzie et al., 1996; Wu et al., 2014; Xia and Wu, 2013). The coordina- tion environment and Y–(O/N) bond distance data of each Y(O/N)n polyhedra are given in Table 6 (Wu et al., 2014). The Y(O/N)n polyhedra are linked by sharing edges between them and further linking with Si2O5N2 ditetrahedral units is achieved by sharing an O/N atom.PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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