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78 Handbook on the Physics and Chemistry of Rare Earths 1 0.5 0 1 0.5 0 200 ex. em A B 300 400 500 600 Wavelength (nm) FIG. 77 Excitation and emission spectra of (A) Ba3(La0.90Tb0.10)Na(PO4)3F and (B) (Ba0.99Eu0.01)3(La0.88Tb0.12)Na(PO4)3F. Redrawn from Jiao, M., Guo, N., Lu€, W., Jia, Y., Lv, W., Zhao, Q., Shao, B., You, H., 2013. Tunable blue-green-emitting Ba3LaNa(PO4)3F:Eu2+, Tb3+ phosphor with energy transfer for near-UV white LEDs. Inorg. Chem. 52, 10340–10346. In (Ba0.99Eu0.01)3(La0.88Tb0.12)Na(PO4)3F, the broad absorption band extending from 240 to 450 nm mainly consists of contributions from Eu2+ (Fig. 77B). Besides, the phosphor shows not only a broad blue emission band (Eu2+) but also an intense green emission (Tb3+). This is a typical example of a Eu2+-to-Tb3+ energy transfer. The emission color of Ba3LaNa(PO4)3F:Eu2+, Tb3+ can be easily modulated from blue to green by adjusting the relative ratio of Eu2+/Tb3+ content. 5 SYNTHESIS METHODS OF WHITE LED PHOSPHORS Typical rare earth-doped phosphors for white LEDs are classified in four groups of compounds: oxides, nitrides (oxynitrides), sulfides (oxysulfides), and halides (oxyhalides). Popular synthesis methods for each group are presented in this section. It is noteworthy that most phosphors are synthesized by reacting a mixture of raw materials containing the constituting elements of the phosphor at high temperature. The luminescent properties of the phosphors are usually improved by optimizing several parameters such as the reaction temperature and time, mixing ratio, quality and nature of the raw materials, as well as by adding reaction acceleration reagents. 5.1 Oxides 5.1.1 Solid-State Reactions The solid-state reaction method is the most popular way of synthesizing phosphors not only oxides but also sulfides and (oxy)nitrides. Most of com- mercially available phosphors are synthesized by this method. Normalized intensity (a.u.)PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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