Influence of oxygen substoichiometry on the dielectric properties of BaTiO_3-δ nanoceramics obtained by spark plasma sintering

Oxygen-deficient BaTiO_3-δ nanoceramics were prepared by spark plasma sintering. Partially reduced raw nanopowders led to unusual dielectric properties. A short postsintering treatment was performed to reach a high ε_r/tan δ, which makes them attractive for industrial applications such as low-frequency capacitors. Surprisingly, our ceramics also remained black even after annealing for 5 days at 850°C in air, indicating the presence of barriers against oxygen diffusion. This exceptional behavior in pure barium titanate was consistent with a core–shell structure made of semiconductive grains and insulating grain boundaries, due to the presence of Ti³⁺/Ti⁴⁺ and oxygen vacancies.