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Rare-earth-doped oxynitride or nitride compounds have been reported to be luminescent and may then serve as new phosphors with good thermal and chemical stabilities. In this work, we report the photoluminescence (PL) spectra of europium-, terbium-, and praseodymium-doped Ca-␣-SiAlON ceramics. The highly dense ceramics were prepared by hot pressing at 1750°C for 1 h under 20 MPa in a nitrogen atmosphere. Europium-doped Ca-␣-SiAlON displayed a single broad emission band peaking at ‫؍‬ 550 -590 nm depending on the europium concentration. The emission bands in the spectra of europium-doped Ca-␣-SiAlONs were assigned to the allowed transition of Eu 2؉ from the lowest crystal field component of 4f 6 5d to 8 S 7/2 (4f 7 ) ground-state level. The emission spectra of terbium-and praseodymium-doped Ca-␣-SiAlON ceramics both consisted of several sharp lines, which were attributed to the 5 D 4 3 7 F j (j ‫؍‬ 3, 4, 5, 6) transitions of Tb 3؉ and 3 P 0 3 3 H j (j ‫؍‬ 3, 4, 5) transitions of Pr 3؉ , respectively. In particular, the terbium-doped ␣-SiAlON ceramics showed a strong green emission among these phosphors.

New host lattice materials whose red phosphors for white LEDs have been investigated in the ternary system Ca 3 N 2 -AlN-Si 3 N 4 , just as Ca 2 Si 5 N 8 and CaSiN 2 : Eu were found in the binary system Ca 3 N 2 -Si 3 N 4 . A new red phosphor of CaAlSiN 3 : Eu which is effectively excited by blue-GaN and near UV-GaInN LED chips has been synthesized at 1600 °C for 2 h and subsequently at 1800 °C for 2 h under nitrogen pressure of 1 MPa. The host-compound has an orthorhombic structure with the space group Cmc2 1 (No. 36), which is isotypic with LiSi 2 N 3 and NaSi 2 N 3 . The red phosphor showed the emission peak around 650 nm which was assinged to 5d → 4f of Eu 2+ ion, and its color coordinates were estimated to be 0.667 and 0.327. The optimum concentration of Eu 2+ ion was 1.6 mol%. The phosphor also had a high chemical stability, high quantum output, and especially a good thermal property compared to the other phosphors, Ca 2 Si 5 N 8 :Eu 2+ and CaSiN 2 :Eu 2+ . CaAlSiN 3 :Eu 2+ maintained 83% of the initial efficiency above 150 °C.

This contribution reports on luminescence properties of divalent ytterbium in alpha-SiAlON at room temperature. Ytterbium-doped alpha-SiAlON powders, with the compositions of (M(1-2x/v)Yb(x))(m/v)Si(12-m-n)Al(m+n)O(n)N(16-n) (M = Ca, Li, Mg, and Y, v is the valency of M, 0.002 < or = x < or = 0.10, 0.5 < or = m = 2n < or = 3.5), were synthesized by sintering at 1700 degrees C for 2 h under 0.5 MPa N2. A single, intense, broad emission band, centered at 549 nm, is observed due to the electronic transitions from the excited state 4f(13)5d to the ground state 4f14 of Yb2+. The luminescence of Yb2+ in alpha-SiAlON occurs at relatively low energy, which is attributable to the large crystal field splitting and nephelauxetic effect due to the nitrogen-rich coordination of Yb2+. The dependence of luminescence properties on the Yb2+ concentration, chemical composition, and annealing is discussed. It is suggested that this novel green phosphor could be applied in white light-emitting diodes (LEDs) when combined with a red phosphor and a blue LED.

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A new oxynitride, Ba3Si6O12N2, has been synthesized. The crystal structure has been successfully determined by close collaboration between experiment and first-principles calculation. This compound doped with Eu exhibits intense green photoluminescence with high color purity under near-ultraviolet to blue light excitation; in particular, it has much less thermal quenching than (Ba,Sr,Eu)2SiO4. Thus (Ba,Eu)3Si6O12N2 appears promising green phosphor for white LED backlight for display. The atomic/electronic structure is discussed in comparison with Ba3Si6O9N4, which could not become efficient phosphor by doping Eu due to strong thermal quenching at room temperature.

Atomic and electronic structure of nitridoaluminosilicate CaAlSiN 3 (Cmc2 1 , No. 36), a distorted AlNbased wurtzite superstructure with Al and Si disordered on 8b site and Ca occupying 4a site, has been investigated by first-principles pseudopotential method based on density functional theory. The random distribution of Al/Si atoms is treated in two ways: (1) virtual crystal approximation (VCA) with a mixed Al/Si pseudopotential, (2) several assumptions of Al/Si distribution order in the primitive unit cell. Geometry optimization based on the aliovalent VCA reproduces the experimental orthorhombic structure as well as the bond lengths. The calculations of Al/Si-ordered models lead to monoclinic structures that appear sufficiently close to the experimental structure, with the averages of the optimized Al-N/Si-N bond lengths corresponding to the experimental ones. Relative stability among the Al/Si-ordered models appears consistent with Pauling's second crystal rule. The random Al/Si distribution leading to the experimental determination of the system as Cmc2 1 can be explained by crystal symmetry, energetics among the ordered models, and configurational entropy effect. The electronic structure based on the VCA appears similar with those of the Al/Si-ordered models. All the band structures indicate that the system has indirect band gap.

Efficient phosphors for white LEDs have been successfully developed, wherein some Material Design Concepts were utilized to promote our research and development effectively and efficiently. Useful ideas for the development of our red and green phosphors, Sr-rich (Sr,Ca,Eu)AlSiN 3 , (Ba,Eu) 3 Si 6 O 12 N 2 and (Ca,Ce) 3 (Sc,Mg) 2 Si 3 O 12 , are reviewed.