In this study, (Zr_0.5/Sb_0.5)_xTi_1-xO₂ ceramics with x = 0.01, 0.025, and 0.05 were prepared using the solid‒state reaction method. A pure phase of rutile TiO₂ with high dense microstructure with relative densities higher than 96% was detected in all sintered ceramics. The mean grain size was reduced but the dielectric permittivity (e¢) increased. The giant dielectric properties were tested to investigate the possible uses in capacitors and capacitive humidity sensors under various relative humidity (RH) levels ranging from 30%RH to 95%RH. The (Zr_0.5/Sb_0.5)_xTi_1-xO₂ ceramics exhibit a giant e¢ of ~ 4.82‒7.39×10⁴ and low loss tangent (tand~0.031‒0.106 at 1 kHz), presenting attractive giant dielectric properties. This observation was attributed to both intrinsic and extrinsic effects. For humidity sensing properties, the most optimum humidity sensing properties were observed in the ceramic with x = 0.05, with a sensitivity of ~237 %pF/%RH, low hysteresis error (~1.6%), and fast response/recovery time of ~12 s/16 s at 1 kHz. The point defects of   and  were claimed to be active centers for water absorption. Furthermore, impedance spectroscopy analysis revealed that changes in dielectric properties with varying RH levels were also influenced by interfacial polarization at the surface layer and grain boundaries.

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.