DFT calculations and giant dielectric responses in (Ni_1/3Nb_2/3)Ti_1-O₂

The origins of dielectric responses in Ni²⁺ and Nb⁵⁺ co-doped TiO₂ were explored considering intrinsic and extrinsic effects. DFT calculations demonstrated that Ni²⁺ doping induced oxygen vacancies, while Nb⁵⁺ doping generated free electrons. Theoretical predictions indicated complex defect dipoles forming in the rutile structure, contributing to overall dielectric responses. Theoretical calculations also showed a possible linear alignment of Ni²⁺-2Nb⁵⁺ without oxygen vacancies, especially in high doping concentrations. Experimentally, (Ni_1/3Nb_2/3)Ti_1-O₂ ceramics (x = 1%, 2.5%, and 10%) were synthesized. The substantial dielectric response at room temperature, attributed to factors like defect dipoles and grain boundary/surface barrier layer capacitor (GBLC/SBLC) effects, increased with higher doping levels. However, in a temperature range where GBLC/SBLC effects were suppressed, the dielectric response decreased with increased doping, likely due to self-charge compensation between Ni²⁺-2N^b5+. Notably, (Ni_1/3Nb_2/3)Ti_1-O₂ with x = 2.5% exhibited a high dielectric permittivity of 10⁴ and a low loss tangent of 0.029 at 1 kHz. Moreover, the dielectric permittivity changed by less than ±15% (compared to 25 °C) at 150 °C. This work provides an understanding of the origins of dielectric responses in co-doped TiO₂ and optimizes the doping concentration to achieve the best dielectric performance.