The luminescence properties of Eu 2+ -activated alkaline-earth silicon-oxynitrides have been studied. In the BaO-SiO 2 -Si 3 N 4 system, a new BaSi 2 O 2 N 2 compound was obtained having the monoclinic structure with lattice parameters a ) 14.070(4) Å, b ) 7.276(2) Å, c ) 13.181(3) Å, β ) 107.74(6)°. All MSi 2 O 2-δ N 2+2/3δ :Eu 2+ (M ) Ca, Sr, Ba) materials can be efficiently excited in the UV to visible region (370-460 nm), making them attractive as conversion phosphors for LED applications. A blue-green emission at 490-500 is observed for BaSi 2 O 2 N 2 :Eu 2+ , yellow emission at 560 nm for CaSi 2 O 2-δ N 2+2/3δ : Eu 2+ (δ ≈ 0), and a green-yellow emission peaking from 530 to 570 nm for SrSi 2 O 2-δ N 2+2/3δ :Eu 2+ (δ ≈ 1), the position depending on the exact value of δ. BaSi 2 O 2 N 2 :Eu 2+ is the most promising conversion phosphor for white-light LEDs due to its high conversion efficiency for blue light from InGaN-based LEDs related to its very small Stokes shift.

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Detailed results of ab initio band structure calculations for tantalum (oxy)nitrides (TaON and Ta 3 N 5) are reported. The calculations are performed within the framework of density functional theory (DFT). We compare results obtained with the molecular dynamics pseudopotential (PP) approach of the Vienna Ab initio Simulation Program (VASP) and the Full Potential Linearized Augmented Plane Waves method (FP-LAPW) using the WIEN97 program. In agreement with neutron diffraction measurements, we show an ordering of the anions in TaON. The calculations also show that the valence band is composed mainly of the anion 2p orbitals hybridized with Ta 5d states. For TaON the top of the valence band is dominated by N 2p states. The bottom of the conduction band is mainly composed of Ta 5d states. Both TaON and Ta 3 N 5 are semiconductors with calculated indirect band gaps of respectively 1.8 and 1.1 eV (VASP calculations) and 2.0 and 1.2 eV (WIEN97 calculations). Optical diffuse-reflectance spectra show an energy gap of 2.08 eV for Ta 3 N 5 .