DFT calculations and giant dielectric responses in (Ni 1/3 Nb 2/3 ) x Ti 1- x O 2 .

The origins of dielectric responses in Ni 2+ and Nb 5+ co-doped TiO 2 were explored considering intrinsic and extrinsic effects. DFT calculations demonstrated that Ni 2+ doping induced oxygen vacancies, while Nb 5+ 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 Ni 2+ -2Nb 5+ without oxygen vacancies, especially in high doping concentrations. Experimentally, (Ni 1/3 Nb 2/3 ) x Ti 1- x O 2 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 Ni 2+ -2N b5+ . Notably, (Ni 1/3 Nb 2/3 ) x Ti 1- x O 2 with x = 2.5% exhibited a high dielectric permittivity of 10 4 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 TiO 2 and optimizes the doping concentration to achieve the best dielectric performance.