New strategy for optimizing the microstructure and giant dielectric properties of TiO2via acceptor/donor ratio tuning

Abstract

In this study, we investigated how the acceptor/donor doping ratio influences the microstructure and giant dielectric behavior of co-substituted TiO₂ ceramics. Sc_xTa_0.025Ti_0.975−xO₂ ceramics, with Sc³⁺/Ta⁵⁺ ratios of 0.4, 0.8, 1.0, and 2.0, were synthesized via solid-state reactions. All samples crystallized into dense rutile TiO₂, and Raman spectroscopy revealed that increasing the Sc³⁺/Ta⁵⁺ ratio promotes the formation of oxygen vacancies, leading to larger average grain sizes. The dielectric constant (ε′) decreased significantly with higher Sc³⁺/Ta⁵⁺ ratios, and no giant dielectric response was observed for ratios above 1.0. Notably, samples with Sc³⁺/Ta⁵⁺ ratios of 0.4 and 0.8 achieved ε′ values of 5.9 × 10⁴ and 4.8 × 10⁴, respectively, alongside low loss tangents (tan δ) of 0.024 and 0.043 at 1 kHz and 25 °C. These ceramics also exhibited excellent temperature stability, with their ε′ values varying by less than ±15% from −60 to 210 °C—sufficient for X9R capacitor applications. Impedance spectroscopy and nonlinear electrical measurements revealed that the enhanced dielectric performance arises primarily from interfacial polarization effects due to insulating grain boundaries and conductive grains, as further confirmed by X-ray photoelectron spectroscopy. Interestingly, the optimal dielectric properties, commonly reported at an acceptor/donor ratio of 1.0 in other co-doped systems, were not observed in this study. These findings challenge the conventional assumption that a 1 : 1 acceptor/donor ratio is universally optimal for co-doped TiO₂ ceramics. This work provides a new strategy for enhancing dielectric performance by adjusting the doping ratio in systems dominated by extrinsic mechanisms such as IBLC.