Structural and Complex AC Impedance Spectroscopic Studies of A2CoNbO6 (A: Sr, Ba) Ordered Double Perovskites.

Abstract Powder X-ray diffraction has been employed to study the crystal structures of Sr 2 CoNbO 6 ( SCNO ) and Ba 2 CoNbO 6 ( BCNO ) double perovskites. Rietveld fit to the X-ray diffraction data showed that Ba 2 CoNbO 6 perovskites was monoclinic with space group P 2 1 / n whereas Sr 2 CoNbO 6 was found to be tetragonal with space group I 4/ 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.