Impedance and dielectric spectroscopy of functional materials: a critical evaluation of the two techniques

Impedance and dielectric spectroscopies are closely related techniques for measuring the electrical properties of materials. The techniques differ in two ways. First, impedance measurements are usually made over several decades of frequency (i.e. broadband) whereas most dielectric measurements are made at fixed frequency. Second, time constants that control semicircles in impedance complex plane plots and peaks in permittivity or tan δ spectroscopic plots are not the same. Differences between the techniques are confined to data analysis procedures and interpretation since they use similar instrumentation for measurements and data collection. In impedance data, time constants represent conducting components and parallel resistance-capacitance (RC) combinations; in permittivity data, they represent dielectric processes and series RC combinations. Using broadband data, it is possible to (i) determine the best equivalent circuit to fit experimental data, (ii) unambiguously evaluate and assign resistance, capacitance, and time constant parameters to regions of the material being measured and (iii) quantify departures from ideality using constant phase elements, CPEs. Using fixed frequency, variable temperature data in either impedance or dielectric methodologies, it is possible to detect the presence of different electrical components that contribute to a data set. However, it is not possible to separate the effects of frequency and temperature in terms of equivalent circuits, nor to deconvolute, parametrise, quantify, and assign the results to different regions of the sample. The advantages of using broadband measurements are highlighted with two examples: calcium copper titanate, CCTO which is often, erroneously, described as a giant or colossal dielectric; lead magnesium niobate, PMN, the classic relaxor ferroelectric whose characteristic properties are controlled entirely by the presence of non-ideality, represented by a CPE, in its equivalent circuit.