Crystal-structure and electron-density analyses of the perovskitetype 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 Eg than Eg of BaNbO2N. The wider Eg of SrNbO2N compared to BaNbO2N is attributable to the smaller Nb( O,N)Nb angles of SrNbO2N.