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Rare Earth-Doped Phosphors for White LEDs Chapter 278 57 x =1 0 1 0.5 0 500 FIG. 53 Excitation and emission spectra of SrxCa1xAlSiN3. Redrawn from Watanabe, H., Kijima, N., 2009. Crystal structure and luminescence properties of SrxCa1xAlSiN3:Eu2+ mixed nitride phosphors. J. Alloy Compd. 475, 434–439. 4.2.4.2 AE2Si5N8:Eu2+ (AE5Ca, Sr, and Ba) Phosphors AE2Si5N8:Eu2+, so-called 2–5–8 phosphors, have been usually used in blue- LED-based white LEDs as a highly efficient conversion red-emitting phos- phors (H€oppe et al., 2000; Li et al., 2006a; Suehiro et al., 2013; Xie et al., 2006a,b,c, 2007, 2010; Zeuner et al., 2009). Schnick’s group first reported the crystal structure of AE2Si5N8 (AE1⁄4Ca, Sr, and Ba) by single crystal structure determination (H€oppe et al., 2004; Schlieper et al., 1995). Table 7 summarizes the corresponding crystal data. Ca2Si5N8 has a monoclinic struc- ture with space group Cc, whereas both Sr2Si5N8 and Ba2Si5N8 have an orthorhombic structure with space group Pmn21. Although AE2Si5N8 have different crystal structures, the local structure is quite similar; the coordination polyhedron of Si is a SiN4 tetrahedron corner shared with other tetrahedra, providing a 3D framework. Half of the nitride ions are bonded to two Si neighbors while the other half have three Si neighbors (Fig. 54). The coordi- nation numbers of Sr2+ (Ba2+) ions are 8 and 9, respectively, while that of Ca2+ is 7 (Piao et al., 2006; Schlieper et al., 1995). Interestingly, the unit cell volume of Ca2Si5N8 remains almost constant even when the Eu2+ content goes up to 7 mol% (x1⁄40.14 in Ca2xEuxSi5N8) (Li et al., 2006a). This indicates that the Ca2Si5N8 framework is quite rigid and robust. On the other hand, the cell volume of Sr and Ba derivatives decreases linearly with increasing Eu2+ content in AE2xEuxSi5N8 (AE 1⁄4 Sr and Ba, 0.02 x 2.0) although the lattice shrinkage for Ba2Si5N8 is 600 Wavelength (nm) 700 800 Normarized intensity (a.u.)PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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