The excellent giant dielectric properties (ExGDPs) are represented in the isovalent–Zr⁴⁺/pentavalent–Ta⁵⁺ ions co–doped TiO₂ with different co–doping percentages (x%ZrTTO). The dopants were dispersed homogeneously in a highly compact–grained ZrTTO microstructure. The mean grain size and cell parameters with bond lengths slightly enlarged as x% increased. The (1%–5%) ZrTTO oxides exhibited ultra–low tanδ values of 0.004–0.016 with the giant dielectric permittivity (ε′~2.7–3.7 × 10⁴); while the ε′ of the 5%ZrTTO was slightly dependent on the temperature ranging from −60 to 200 ℃, following the temperature dependence requirement for application in the X7/8/9R capacitors. Impedance spectroscopy showed a very large resistance of the grain boundaries. The dielectric properties of the 1%ZrTTO were strongly dependent on the applied DC electric field, indicating the dominant internal barrier layer capacitor (IBLC) effect. However, the dielectric properties of the 5%ZrTTO were nearly independent on the applied DC electric field up to 30 V/mm, which was primarily resulted from electron localization in defect dipoles. Therefore, the ExGDPs of the x%ZrTTO were attributed to the combined effects of the IBLC and localized–electron defect–dipoles related to oxygen vacancies (Ti⁴⁺·e⁻−V_O^••−e⁻·Ti⁴⁺ and $3\left(\mathrm{T}{\mathrm{i}}^{4+}\cdot {\mathrm{e}}^{-}\right)-V_{\mathrm{O}}^{••}-{\mathrm{T}\mathrm{a}}_{\mathrm{T}\mathrm{i}}^{•}$) and $\mathrm{T}{\mathrm{i}}^{4+}\cdot {\mathrm{e}}^{-}-{\mathrm{T}\mathrm{a}}_{\mathrm{T}\mathrm{i}}^{•}$.

The giant dielectric behavior of CaCu₃Ti₄O₁₂ (CCTO) has been widely investigated owing to its potential applications in electronics; however, the loss tangent (tanδ) of this material is too large for many applications. A partial substitution of CCTO ceramics with either Al³⁺ or Ta⁵⁺ ions generally results in poorer nonlinear properties and an associated increase in tanδ (to ~0.29-1.15). However, first-principles calculations showed that self-charge compensation occurs between these two dopant ions when co-doped into Ti⁴⁺ sites, which can improve the electrical properties of the grain boundary (GB). Surprisingly, in this study, a greatly enhanced breakdown electric field (~200-6588 V/cm) and nonlinear coefficient (~4.8-15.2) with a significantly reduced tanδ (~0.010-0.036) were obtained by simultaneous partial substitution of CCTO with acceptor-donor (Al³⁺, Ta⁵⁺) dopants to produce (Al³⁺, Ta⁵⁺)-CCTO ceramics. The reduced tanδ and improved nonlinear properties were attributed to the synergistic effects of the co-dopants in the doped CCTO structure. The significant reduction in the mean grain size of the (Al³⁺, Ta⁵⁺)-CCTO ceramics compared to pure CCTO was mainly because of the Ta⁵⁺ ions. Accordingly, the increased GB density due to the reduced grain size and the larger Schottky barrier height (Φ_b) at the GBs of the co-doped CCTO ceramics were the main reasons for the greatly increased GB resistance, improved nonlinear properties, and reduced tanδ values compared to pure and single-doped CCTO. In addition, high dielectric constant values (ε′ ≈ (0.52-2.7) × 10⁴) were obtained. A fine-grained microstructure with highly insulating GBs was obtained by Ta⁵⁺ doping, while co-doping with Ta⁵⁺ and Al³⁺ resulted in a high Φ_b. The obtained results are expected to provide useful guidelines for developing new giant dielectric ceramics with excellent dielectric properties.