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 Å. SrTaO2N and CaTaO2N are distorted by octahedral tilting, showing noticeably smaller Ta−O/N distances of approximately 2.02 Å. 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, κ ≈ 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.