Structural and complex AC impedance spectroscopic studies of A2CoNbO6 (A = Sr, Ba) ordered double perovskites

Powder X-ray diffraction has been employed to study the crystal structures of Sr2CoNbO6 (SCNO) and Ba2CoNbO6 (BCNO) double perovskites. Rietveld fit to the X-ray diffraction data showed that Ba2CoNbO6 perovskites was monoclinic with space group P2(1)/n whereas Sr2CoNbO6 was found to be tetragonal with space group 14/m. Like other cobalt based perovskites, both materials exhibit high values of dielectric constant at room temperature and low frequencies. Room temperature impedance and modulus spectra, measured over the 1 Hz to 10 MHz, reveal two relaxation processes with different relaxation times which were attributed to the grain and grain boundaries. (C) 2011 Elsevier Masson SAS. All rights reserved.