Electrode-spacer and other effects on the validity of the Kramers–Kronig relations and the fittings to the permittivity and electrical modulus spectra

We argue that use of certain electrode assemblies adversely affects the measured spectra of both ɛ′ and tan δ (=ɛ′′/ɛ′) and thus violates an aspect of the Kramers–Kronig relations. The values of ɛs, ɛ∞ and tan δ measured are not the true values of the sample and their analysis leads to questionable conclusions. Some of the data in the literature can be corrected for this effect if the ɛ′-spectra are available. Such electrode assemblies also yield erroneous values of tan δ measured for studies of structural relaxation. Also, the electrical modulus, M* = 1/ɛ*, spectra of ion-containing materials violate the Kramers–Kronig relations because M′ and M″ values contain contributions from electrode polarization, in addition to the electrode spacer effects. Fitting of the relaxation functions to such data or analyses of the spectral-differences may be misleading.