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Rare Earth-Doped Phosphors for White LEDs Chapter 278 15 5d eg t2g 4f Free ion Nephelauxetic Weak Strong Distorted effect cubic field cubic field field Schematic diagram of energies of 5d levels and emissions of rare earth ions such as Eu2+ or Ce3+ affected by bonding (nephelauxetic effect) and crystal-field splitting in various environments. increases the covalency of bonding. Interelectronic repulsion decreases and the nephelauxetic effect increases. The crystal-field splitting becomes larger as a consequence of the higher charge of nitride. The latter allows the forma- tion of new compounds where cations show higher oxidation states than in oxides. These effects induce important changes in properties. 4 PHOSPHOR MATERIALS FOR WHITE LEDs 4.1 Oxides Oxide phosphors are presently the most frequently available phosphors. Since they can be synthesized at a lower temperature compared with nitride phos- phors, control of the morphology of the powders is easy when using conven- tional ceramic synthetic methods. Red shifting emission and excitation spectra are difficult for these phosphors due to reduced nephelauxetic effect. How- ever, if an appropriate crystal matrix is carefully chosen, the red shift can be obtained with an oxide matrix. In general, the production costs of oxide phosphors are less than those of conventional nitride phosphors because of inexpensive starting materials and low synthesis temperatures. 4.1.1 Yellow-Emitting Phosphors 4.1.1.1 Y3Al5O12:Ce3+ Phosphors Ce3+-activated yttrium aluminum garnet YAG:Ce3+ has been often used as a commercial yellow phosphor in phosphor-converted white LEDs (Chen et al., 2010a; Tamaki and Murazaki, 2007; Xie et al., 2011). The YAG:Ce3+ phosphor has a strong optical absorption band in the blue which well matches with the emission band of InGaN-based blue-LED chips, and it exhibits a strong and broad yellow emission. As shown in Fig. 7, the excitation spectrum FIG. 6PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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