Structural and complex ac impedance spectroscopic studies of A/sub 2/CoNbO/sub 6/ (a= sr, ba) ordered double perovskites

Powder X-ray diffraction (XRD) was employed to study the crystal structures of Sr/sub 2/CoNbO/sub 6/ (SCNO) and A/sub 2/CoNbO/sub 6/ Ba/sub 2/CoNbO/sub 6/ (BCNO) double perovskites. Rietveld fit to the X-ray diffraction data showed that Ba/sub 2/CoNbO/sub 6/ perovskites was monoclinic with space group P21/n whereas Sr/sub 2/CoNbO/sub 6/ was found to be tetragonal with space group I4/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 1Hz to 1MHz, revealed two relaxation processes with different relaxation times which were attributed to the grain and grain boundaries. Frequency dependence of real part of impedance is shown.(Orig./A.B.)