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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 .

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