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18 Handbook on the Physics and Chemistry of Rare Earths FIG. 9 The crystal structure of YAG (A), coordination environment of YO8 polyhedron in YAG lattice (B), and schematic of oxygen coordination and interatomic distances for a portion of YAG structure (C). The unit of bond distance is nm. Pictures were drawn based on the structure data obtained from the database (ICSD #16825). YAG:Ce3+ and explained it based on the crystal structure of YAG. YAG has a cubic lattice with a space group of Ia3d, and the lattice constants, a 1⁄4 1.20136(9) nm (Chenavas et al., 1978; Meagher, 1975; Pavese et al., 1995; Xie et al., 2011) (Fig. 9A). Y3+ ions adopt eightfold oxygen coordination, and the YO8 dodeca- hedron can be represented as a distorted cube in between a regular cube and an antiprism (Fig. 9B) (Kanke and Navrotsky, 1998). Al3+ occupies two inde- pendent sites within an AlO4 tetrahedron and an AlO6 octahedron, which share edges with the YO8 dodecahedron, whereas they do not share any edge with themselves (Fig. 9C). When an ion substitutes the Y3+ site, the distance of Y–O in the lattice is strongly influenced by the ionic radius of the ion, while the Al–O distance does not change so much (Kanke and Navrotsky, 1998; Wu et al., 2007a,b). In other words, the change in the O–O distance in edges shared with another YO8 dodecahedron is larger than when the edge is shared with the AlO4 tetrahedron. Therefore, the distortion of the YO8 dodecahedron is relaxed by substituting smaller ions in the Y3+ site, which reduces the crystal-field splitting of 5d excited level of Ce3+. In white LED applications, the effect of thermal stability of phosphors on temperature quenching is significantly important because it has great influencePDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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