Local structure and electronic properties of BaTaO2N with perovskite-type structure

First-principles calculation based on density-functional theory in the pseudo-potential approach have been performed for the total energy and crystal structure of BaTaO2N. The calculations indicate a random occupation of the anionic positions by O and N in a cubic structure, in agreement with neutron diffraction measurements and infrared spectra. The local symmetry in the crystal is broken, maintaining a space group Pm (3) over barm, as used in structure refinement, which represents only the statistically averaged result. The calculations also indicate displacive disordering in the crystal. The average Ta-N distance is smaller (2.003 Angstrom), while the average Ta-O distance becomes larger (2.089 Angstrom). The local relaxation of the atoms has an influence on the electronic structure, especially on the energy gap. BaTaO2N is calculated to be a semiconductor with an energy gap of about 0.5 eV. The upper part of the valence band is dominated by N 2p states, while O 2p states are mainly in the lower part. The conduction band is dominated by Ta 5d states. (C) 2002 Elsevier Science Ltd. All rights reserved.