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38 Handbook on the Physics and Chemistry of Rare Earths 4.1.3.2 Olivine-Type NaMgPO4:Eu2+ Phosphors Phosphate compounds have also been considered as excellent host materials of phosphors for white LEDs due to their low material cost, excellent thermal and hydrolytic stabilities, and charge stabilization (Tang et al., 2007; Wu and Wang, 2007; Wu et al., 2006, 2007a,b). Recently, a red-emitting phosphate phosphor excitable by a blue light, olivine-type NaMgPO4:Eu2+, was reported by Kim et al. (2013a). NaMgPO4 has an orthorhombic structure with space group Pnma, a1⁄41.03701(8) nm, b1⁄40.61729(5) nm, c1⁄40.49347(4) nm, and V1⁄40.31284(4) nm3. The structure is built up of PO4 tetrahedra and MgO6 octahedra that connect each other by corner sharing and edge sharing, forming a 3D framework defining channels along the b-axis where Na+ ions can be located, as illustrated in Fig. 33. From the viewpoint of ionic radius, it is reasonable to take into account that Eu2+ ions substitute the Na+ sites in NaMgPO4:Eu2+. The average Na–O bond distance in NaO6 is 0.2360 nm. Fig. 34 shows the excitation and emission spectra of (Na0.975Eu0.025) MgPO4 (Kim et al., 2013a). The phosphor exhibits a strong broad red emis- sion band centered at 628 nm under excitation at 450 nm. In addition, the excitation spectrum consists of a broad band covering the region from UV to visible, indicating that the phosphor is suitable as wavelength converter excited by any wavelength in this range. The internal quantum efficiency of NaMgPO4:Eu2+ at an excitation wavelength of 450 nm is 81%. The phosphor also has excellent thermal stability with respect to temperature quenching as shown in Fig. 35 where the emission intensities are normalized with respect to the value at 25°C. With increasing temperature up to 150°C, the emission intensity of NaMgPO4:Eu2+ decreases to 86%. FIG. 33 a b c PO4 MgO6 Na 0.2347(5) 0.2335(5) 0.2397(5) 0.2397(5) 0.2335(5) 0.2347(5) Crystal structure of olivine-type NaMgPO4. The unit of bond distance is nm. Pictures were drawn based on the structural data obtained from Kim, S.W., Hasegawa, T., Ishigaki, T., Uematsu, K., Toda, K., Sato, M., 2013. Efficient red emission of blue-light excitable new structure type NaMgPO4:Eu2+ phosphor. ECS Solid State Lett. 2, R49–R51.PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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