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 Ti 3+ /Ti 4+ and oxygen vacancies.
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Voisin et al. (2013) studied this question.
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