Crystal-structure and electron-density analyses of the perovskite-type oxynitrides BaNbO₂N and SrNbO₂N through synchrotron X-ray powder diffraction

Abstract

Crystal structure and electron-density distribution of the perovskite-type oxynitrides BaNbO₂N and SrNbO₂N have been analyzed by the synchrotron X-ray powder diffraction, Rietveld and maximum-entropy methods. In both BaNbO₂N and SrNbO₂N, the electron-density levels along the Nb–(O,N) bonds are significantly higher than those along the Ba–(O,N) and Sr–(O,N) bonds, which indicates higher covalency of the Nb–(O,N) bonds. The Nb–(O,N) bonds in SrNbO₂N have higher electron-density levels than that in BaNbO₂N, which suggests higher covalency of Nb–(O,N) bonds in SrNbO₂N. The higher covalency of SrNbO₂N cannot explain its wider band gap E_g than E_g of BaNbO₂N. The wider E_g of SrNbO₂N compared to BaNbO₂N is attributable to the smaller Nb–(O,N)–Nb′ angles of SrNbO₂N.