Origin of the colossal permittivity of (Nb plus In) co-doped rutile ceramics by wide-range dielectric spectroscopy

Searching for giant dipolar defects, suggested as the explanation of the colossal permittivity (CP) in (Nb + In) co-doped rutile, we have recently published (J. Appl. Phys. 119, 154105 (2016) and Phys. Rev. Mat., in press) temperature-dependent dielectric spectra from sub-Hz to THz range (including 4-point DC conductivity) of several (Nb + In) co-doped rutile ceramics. Using the model of two leaky capacitor in series below similar to 1 MHz, we estimated the depletion layer thickness and conductivity. The spectra below similar to 10 GHz were fitted with two Cole-Cole relaxations obeying the Arrhenius law, assigned to the effect of depletion layers and grain boundaries. In this paper, using effective medium models, we have for the first time modelled the spectra as a composite of semiconducting grains and substantially less-conducting grain boundaries and near-electrode depletion layers. The CP effect has been fully explained by the combination of surface and internal barrier-layer capacitor effects, without any measurable contribution from the dipolar defects.