Dear Ch Thirmal,

For polycrystals the two relaxations are usually attributed to charge conduction in grain cores and grain boundaries. You can confirm that by comparing the values of capacitance associated to each peak, or through examination of the imaginary part of the electric modulus vs frequency plot. A relaxation related to grain boundaries should not result in a peak in M'' spectra.

Also, the two semicircles in Nyquist plots are exactly the same relaxations you are observing in Z'' vs f plots.

For more information read these references:

J. Electroanal. Chem 74, 125 (1976)

J. Phys. Chem. Solids 12, 1491 (1994)

Regards

Thank you sir for your answer,

And how do i know the type of charge carriers (electrons or protons or ions) ?

Is there any method to find out the this origin of conductivity through impedance spectra?

As far as I know impedance spectroscopy is not enough to determine the type of charge carriers. Other techniques are needed in order to do that. You can, however, obtain more details about the conductivity, such as carriers mobility. See for example:

Phys. Rev B 75, 075307 (2007)

Dear Ch Thirmal,

Journal of Molecular Structure, Volumes 1056–1057, 6 January 2014, Pages 152–156.

I hope this might be useful one for you.