IMPEDANCE AND DIELECTRIC-SPECTROSCOPY REVISITED - DISTINGUISHING LOCALIZED RELAXATION FROM LONG-RANGE CONDUCTIVITY

The advantages of plotting a.c. data in terms of impedance, electric modulus and dissipation factor simultaneously are illustrated. Complex impedance is generally employed for ionic conductors because it can easily distinguish between bulk and grain boundary effects. However, comparison with the modulus and dissipation factor data allows easier interpretation of the microscopic processes responsible for the measured a.c. response. In particular, the difference between localized (i.e. dielectric relaxation) and non-localized conduction (i.e. long range conductivity) processes within the bulk of the material may be discerned by the presence or the absence of a peak in the imaginary modulus versus frequency plot. Similarly, the absence or presence of a peak in the imaginary impedance versus frequency plot can be correlated to space charge effects and non-localized conductivity. Long-range conductivity results in nearly complete impedance semicircles but no frequency dispersion in the permittivity while localized conductivity is reflected in a frequency dependent permittivity but no measurable conductance. The degree to which these assignments may be made is related to the dielectric relaxation ratio (r = epsilon(s)/epsilon infinity) and the differences between the time constants of the different relaxation processes present in the material being examined.