ARTICLE IN PRESS R-J.Xle,N.Hirosaki/Sclence and Technology of Adeanced Materials() Alternatively.nitride com ounds can also be divided engineers.For rare-earth ions (ie..Eu2 and Ce)with the 5d electrons unshielded from the crystal field by the 5 ents i binary,(ii)ter (quaternary and 5p electrons wher in the excite te,the spe BN. and AIN not bary ent m asily d as hen a for phosphors in white LEDs because they do not have bond length,site size,crystal-field strength,etc.).Becaus of the higher formal charge of N compared with 7-10 The he nepe covalent nitride iicon-based nitride of gravity of the sd states is shifted to lower are interesting because of their unique and rigid crystal ab ty of s able environmen oxynitride earth ions to provide u ping c and emission wavelengths than their oxide Furthermore,the Stokes shift becomes smaller in a rigid the preparation and crysta silicon-base etwork of SIN tetrahedr tena sialons is formed by the integ tion of nit gen in silicates A variety of oxynitride and nitride materials with aluminosilicat Com with the romising umine nt prope erties have been discovered dly [1-1 9 3].In t of thes addition to these nitride compounds oxynitrides (i.e. oxon 3.1.Blue-emitting phosphors mbind with aluminum resp vely.Therefore.m to oxosilicate and red phosphors to create white light when UV or near structure: oxyni and t(NUV)LED is used.Although a large numbe omp hedra.The degree of condensation in the netw ork of Six nal degradation is a serious problem if they are used in tetrahedra is simply evaluated by the ratio 「44D. Ce s to b ging atom the ate oxynitr pho S ratio n a h indicates that nitrides have a high degree of condensatior white LEDs.In the following.three types of blue-emitting due to the fact the and he phospho atoms O: xyger 1 ides are generall connected with two(N thre Al)N.O was re atoms such as in BaSiN Sr Ba)[24,25 eque (spac rdin ted h (N.O) elements result in the extraordinary chemical and ther vielding an Al(Si.Al)(N,O)network (see Fig.1).The La stability of silicon-based oxynitride and nitride materials caCanmodhedntunncheiendhng re along the and lumings ence of silicon-based oxynitride and nitride phosphors et al.1]reported the luminescence of Ce+-doped JEM As shown in Fig 2,the emission spectrum ,sulfide of JEM:C tending nm nhors is at oxy The road excitatic m extending from 200 to 450m realizing white LEDs has greatly catalyzed the research and due to the 4f. .5d electronic transition of Ce Both ment of oxynitride and nitride pho spectra redshifted whe value Please cite this article as:R.Xie.N.Hirosaki,Sci.Technol.Adv.Mater .(2007.doi10.1016 j.stam2007.08.00Alternatively, nitride compounds can also be divided into the following groups depending on the number of elements included: (i) binary, (ii) ternary, (iii) quaternary, and (iv) multinary. Binary covalent nitrides, such as GaN, BN, and AlN, cannot be easily considered as host lattices for phosphors in white LEDs because they do not have suitable crystal sites for activators [13], although some of them show interesting luminescence properties in thin-film form [7–10]. The ternary, quaternary, and multinary covalent nitride compounds, typically silicon-based nitrides, are interesting because of their unique and rigid crystal structures, availability of suitable crystal sites for activators, and their structural versatility, which enable the doping of rare-earth ions to provide useful photoluminescence. Schnick and coworkers [21–28] extensively investigated the preparation and crystal structures of silicon-based oxynitride and nitride compounds. A new class of materials consisting of nitridosilicates, nitridoaluminosilicates, and sialons is formed by the integration of nitrogen in silicates or aluminosilicates. Compared with the well-known oxosilicates, the newly developed nitrides exhibit a much wider range of structural complexity and flexibility, forming a large family of multiternary compounds. In addition to these nitride compounds, oxynitrides (i.e., oxonitridosilicates and oxonitridoaluminosilicates) are derived from oxosilicates and oxoaluminosilicates by exchanges of oxygen with nitrogen and of silicon with aluminum, respectively. Therefore, similar to oxosilicates, the structures of silicon-based oxynitride and nitride compounds are generally built up of highly condensed networks constructed from linked SiX4 (X=O, N) tetra￾hedra. The degree of condensation in the network of SiX4 tetrahedra is simply evaluated by the ratio of tetrahedral Si centers to bridging atoms X. In oxosilicates the Si:X ratio reaches a maximum of 0.5 in SiO2, while in nitrides the Si:X ratio may vary in a broad range of 0.25–0.75. This indicates that nitrides have a high degree of condensation due to the fact that the structural possibilities in oxosilicates are limited to terminal oxygen atoms and simple bridging O[2] atoms, whereas the nitrogen atoms in nitrides are generally connected with two (N[2]), three (N[3]), even four (N[4]) silicon atoms such as in BaSi7N10 [23] and MYbSi4N7 (M ¼ Sr, Ba) [24,25]. Consequently, the highly condensed SiN4-based networks and the high stability of the chemical bonding between the constituent elements result in the extraordinary chemical and thermal stability of silicon-based oxynitride and nitride materials. 3. Structure and luminescence of silicon-based oxynitride and nitride phosphors Compared with oxide-, boride-, sulfide-, or phosphate￾based phosphors, the study of oxynitride and nitride phosphors is at a very early stage. The possibility of realizing white LEDs has greatly catalyzed the research and development of oxynitride and nitride phosphors, and they are receiving significant attention from both scientists and engineers. For rare-earth ions (i.e., Eu2+ and Ce3+) with the 5d electrons unshielded from the crystal field by the 5s and 5p electrons when in the excited state, the spectral properties are strongly affected by the surrounding environment (e.g., symmetry, covalence, coordination, bond length, site size, crystal-field strength, etc.). Because of the higher formal charge of N3 compared with O2 and the nephelauxetic effect (covalence), the crystal-field split￾ting of the 5d levels of rare earths is larger and the center of gravity of the 5d states is shifted to lower energies (i.e., longer wavelength) than in an analogous oxygen environment. Consequently, silicon-based oxynitride and nitride phosphors are anticipated to show longer excitation and emission wavelengths than their oxide counterparts. Furthermore, the Stokes shift becomes smaller in a rigid lattice with a more extended network of SiN4 tetrahedra. A small Stokes shift leads to high conversion efficiency and small thermal quenching of phosphors. A variety of oxynitride and nitride materials with promising luminescent properties have been discovered recently [11–14,16–19,29–43]. In this section, we will review the structure and luminescence of these rare-earth-doped oxynitride and nitride phosphors. 3.1. Blue-emitting phosphors A blue-emitting phosphor must be combined with green and red phosphors to create white light when UV or near ultraviolet (NUV) LED is used. Although a large number of oxide-based phosphors emit an intense blue color under UV or NUV light excitation, the high thermal quenching or thermal degradation is a serious problem if they are used in white LEDs (e.g., BaMgAl10O17:Eu2+ [44]). Ce3+- or Eu2+-activated oxynitride blue phosphors undergo little thermal degradation and have strong absorption of UV or NUV light, enabling them to be alternative candidates for white LEDs. In the following, three types of blue-emitting oxynitride phosphor (i.e., LaAl(Si6zAlz)N10zOz:Ce3+, a-sialon:Ce3+, and (Y,La)-Si–O–N:Ce3+) will be described. The preparation and crystal structure of a JEM phase with chemical formula LaAl(Si6zAlz)N10zOz was re￾ported by Grins et al. [45]. JEM has an orthorhombic structure (space group Pbcn) with a ¼ 9.4303 A˚ , b ¼ 9.7689 A˚ , and c ¼ 8.9386 A˚ . The Al atoms and (Si, Al) atoms are tetrahedrally coordinated by (N, O) atoms, yielding an Al(Si,Al)6(N,O)10 3 network (see Fig. 1). The La atoms are accommodated in tunnels extending along the [0 0 1] direction and are irregularly coordinated by seven (N, O) atoms at an average distance of 2.70 A˚ . Hirosaki et al. [11] reported the luminescence of Ce3+-doped JEM. As shown in Fig. 2, the emission spectrum of JEM:Ce3+ displays a broad band extending from 400 to 700 nm under 368 nm excitation, with a peak located at 475 nm. The broad excitation spectrum extending from 200 to 450 nm is due to the 4f-5d electronic transition of Ce3+. Both spectra are redshifted when the concentration of Ce3+ or the z value increases, enabling this blue phosphor to be ARTICLE IN PRESS R.-J. Xie, N. Hirosaki / Science and Technology of Advanced Materials ] (]]]]) ]]]–]]] 3 Please cite this article as: R.-J. Xie, N. Hirosaki, Sci. Technol. Adv. Mater. (2007), doi:10.1016/j.stam.2007.08.005