Electrical and Dielectric Properties of Ca-Doped Bi-Deficient Sodium Bismuth Titanate Na0.5Bi0.49−xCaxTiO3−δ (0 ≤ x ≤ 0.08)

Bismuth-deficient sodium bismuth titanate (nominal Na0.5Bi0.49TiO2.985, NB0.49T) presents high oxide ion conductivity, which makes it a potential electrolyte material for intermediate-temperature solid oxide fuel cells. Acceptor doping has been proven an effective approach to enhance the bulk conductivity (σb) of NB0.49T. Here, divalent Ca2+ ions were selected to partially replace Bi3+ on the A-site of NB0.49T, and the temperature and composition dependences of σb and permittivity were investigated. Results showed that Ca2+ doping was effective for enhancing σb of NB0.49T by creating oxygen vacancies. The highest σb (0.006 S·cm−1 at 500 °C) was achieved by 2% Ca2+ doping. Further increase in the doping level decreased σb, which was more pronounced at temperatures below ~350 °C. Most importantly, Ca doping increased the temperature at which the activation energy for bulk conduction changed from ~0.80 eV (at low temperatures) to ~0.40 eV (at high temperatures), and reduced the temperature dependence of permittivity of NB0.49T. Results from the average structural parameters and the local defect associates are discussed. The findings of this work are helpful for understanding the defect and conduction mechanisms for acceptor-doped NB0.49T, and are also useful for developing NBT-based dielectrics with temperature-independent permittivity.