The influence of pentavalent elements (Nb⁵⁺, Ta⁵⁺) with Fe³⁺ on TiO₂ ceramic were investigated. The (Nb₀.₅Fe₀.₅)₀.₁Ti₀.₉O₂ and (Ta₀.₅Fe₀.₅)₀.₁Ti₀.₉O₂ ceramics have been synthesized and realized that the effect of Nb in the system has developed lattice expansion, although Nb and Ta exhibit similar ionic radii. The colossal permittivity value for (Nb₀.₅Fe₀.₅)₀.₁Ti₀.₉O₂ ceramic is about εr ≥ 104 which is slightly higher than the (Ta₀.₅Fe₀.₅)₀.₁Ti₀.₉O₂ ceramic. According to the dielectric analysis, both ceramics exhibit colossal permittivity below 10³ Hz which contributed by the electron-pinned defect-dipole (EPDD) model. However, the low εr values in the high-frequency region are associated with the internal barrier layer capacitor (IBLC) model. In addition, the grain boundary resistance is far greater than the grain resistivity (Rgb≫ Rg) which confirmed the existence of the IBLC effect by equivalent circuit analysis.
