ARTICLE IN PRESS Eu2+-doped sulfides (e.g.,CaS:Eu2[5)).However,these phosphors have eithe )or lo low absorption in the blue-ligh red pho chemical stability and high emission efficiency upon blue Previous studies demonstrated that scribed in the following Schnick et al.[26,27] reported the crystal structures of single crystals of MSisNs(M Ca,Sr,Ba).CazSisNg has a bic lattice with the space group of Pmn2.The local 200 300 400500 600 70D coodneationinte,trnctrc is similar for these ternary Wavelength(nm) neighbors.Each Ca atom in CazSisNs is coordinated to eight or atom 1.0 aic Ca-a-slalon:Eu cence of Eu2-doped BaSisNs was reported by Hoppe 08 sNs(M Ca, 0.6 YAG:Ce splitting and strong nephelauxetic effect.The red phosphor emits an intense orange-red or red color,depending on the 0, Ihe peak emi upwar 03 0 50 100150200 250 30D Temperature (c) which emits a yellow-green color,making it possible to combined lue LED realized:Euyellow phosphor with different emission colors.Furthermore,we have demon- quenching tha VAG-Ce phosphor has lower quenching than cted to of chromatiy n hite LEDs 3.4.Red-emitting phosphors A red-emitting phosphor is usually combined with greer and/or blue phosphors in the case of white LEDs utilizing a UV-,NUV,or blue-LED chip.The search for red phosphors ively. Please cite this articles:RJ.Xie,N.Hirosaki,Sci.Technol.Adv.Mater.(do:.0 which emits a yellow–green color, making it possible to generate daylight light when combined with a blue LED. This indicates that warm-white-to-daylight light can be realized using a single a-sialon:Eu2+ yellow phosphor with different emission colors. Furthermore, we have demon￾strated that the a-sialon:Eu2+ phosphor has lower thermal quenching than YAG:Ce3+, as shown in Fig. 13. The low thermal quenching is expected to lead to a small variation of chromaticity in white LEDs using a-sialon:Eu2+. 3.4. Red-emitting phosphors A red-emitting phosphor is usually combined with green and/or blue phosphors in the case of white LEDs utilizing a UV-, NUV-, or blue-LED chip. The search for red phosphors for use in white LEDs has been mostly concentrated on Eu3+-doped materials (e.g., NaEu(W, Mo)2O8 [60]), and Eu2+-doped sulfides (e.g., CaS:Eu2+ [5]). However, these phosphors have either low absorption in the blue-light range (i.e., oxides) or low chemical stability (i.e., sulfides). It is therefore necessary to develop red phosphors with high chemical stability and high emission efficiency upon blue￾light excitations. Previous studies demonstrated that silicon-based nitride compounds are good host lattices for red luminescent materials [16,41–43], and they are de￾scribed in the following. Schnick et al. [26,27] reported the crystal structures of single crystals of M2Si5N8 (M=Ca, Sr, Ba). Ca2Si5N8 has a monoclinic crystal system with the space group of Cc, whereas both Sr2Si5N8 and Ba2Si5N8 have an orthorhom￾bic lattice with the space group of Pmn21. The local coordination in the structures is similar for these ternary alkaline-earth Si3N4’s; half the nitrogen atoms are con￾nected to two Si neighbors and the other half have three Si neighbors. Each Ca atom in Ca2Si5N8 is coordinated to seven nitrogen atoms, while Sr in Sr2Si5N8 and Ba in Ba2Si5N8 are coordinated to eight or nine nitrogen atoms (see Fig. 14). The average bond length between alkaline￾earth metals and nitrogen is about 2.880 A˚ . The lumines￾cence of Eu2+-doped Ba2Si5N8 was reported by Hoppe et al. [16], and that of Eu2+-doped M2Si5N8 (M ¼ Ca, Sr, Ba) was later reported by Li et al. [41]. The red emission in M2Si5N8:Eu2+ was attributed to the large crystal-field splitting and strong nephelauxetic effect. The red phosphor emits an intense orange-red or red color, depending on the alkaline-earth metal. The peak emission wavelength shifts upward with increasing ionic size of the alkaline-earth ARTICLE IN PRESS Fig. 13. Temperature dependence of emission intensities of Ca-a-sialon:Eu2+ and YAG:Ce3+. Fig. 14. Crystal structure of Sr2Si5N8 viewed along the [0 0 1] direction. The blue, red, and green spheres represent Sr, Si, and N atoms, respectively. Fig. 12. Excitation and emission spectra of Ca0.925Eu0.075Si9Al3ON15. The excitation and monitoring wavelengths are 420 and 581 nm, respectively. 8 R.-J. Xie, N. Hirosaki / Science and Technology of Advanced Materials ] (]]]]) ]]]–]]] Please cite this article as: R.-J. Xie, N. Hirosaki, Sci. Technol. Adv. Mater. (2007), doi:10.1016/j.stam.2007.08.005