Characterization of the Structural, Optical, and Dielectric Properties of Oxynitride Perovskites AMO2N (A = Ba, Sr, Ca; M = Ta, Nb)

The syntheses, crystal structures, electrical properties, and optical absorbance spectra of six perovskite oxynitrides, AMO2N (A = Ba, Sr, Ca; M = Ta, Nb) have been investigated. The average crystal structure of BaTaO2N is a cubic perovskite, with a Ta-O/N distance of 2.056 (angstrom). SrTaO2N and CaTaO2N are distorted by octahedral tilting, showing noticeably smaller TaO/N distances of approximately 2.02 (angstrom). Electron diffraction studies of BaTaO2N are consistent with the simple cubic perovskite crystal structure determined using X-ray powder diffraction methods. Each of the niobium oxynitrides is isostructural with its tantalum analogue, though the Nb-O/N distances are observed to be slightly longer. The optical band gaps are estimated from diffuse reflectance spectra as follows: BaTaO2N, 1.8 eV; SrTaO2N, 2.1 eV; CaTaO2N, 2.4 eV; BaNbO2N, 1.8 eV; SrNbO2N, 1.9 eV; CaNbO2N, 2.1 eV. Impedance spectroscopy was carried out on sintered pellets of the ATaO2N and BaNbO2N to investigate the dielectric and electrical transport properties. The BaNbO2N sample shows metallic-type conductivity apparently from a slight reduction that occurs during sintering. In contrast, the tantalum compounds are semiconductors/insulators with conductivities of ~10-5 S/cm (A = Ba, Sr) and ~10-8 S/cm (A = Ca). Interpretation of the impedance data for BaTaO2N and SrTaO2N reveals that these two compounds have unexpectedly high bulk dielectric constants, (kappa)(almost equal)4900 and 2900, respectively, at room temperature. The dielectric constants of both compounds are frequency dependent and show a relatively weak linear dependence upon temperature with no sign of a phase transition over the temperature range 300-180 K.