Dramatic impact of the TiO2 polymorph on the electrical properties of ‘stoichiometric’ Na0.5Bi0.5TiO3 ceramics prepared by solid-state reaction

Abstract

Bulk conductivity (σ_b) values of nominally stoichiometric Na_0.5Bi_0.5TiO₃ (NBT) prepared by solid-state reaction collated from literature show random variation between 10⁻⁶ to 10⁻³ S cm⁻¹ (at 600 °C). This makes it challenging to obtain reliable and reproducible performances of NBT-based devices, especially as the underlying reason(s) for this variance are not fully understood. Here we report the dramatic impact of the TiO₂ reagent, in particular, the polymorphic form of TiO₂ on the electrical conductivity and conduction mechanism of NBT. Based on our solid-state processing route, NBT ceramics prepared by rutile TiO₂ are ionically conductive, and those prepared by anatase TiO₂ are insulating. The dramatic difference in electrical properties of NBT prepared using rutile and anatase TiO₂ is related to the NBT formation process: the intermediate phase Bi₁₂TiO₂₀ is more stable during formation of NBT in the case of anatase TiO₂, which reduces the volatility of Bi₂O₃ during solid-state reaction. These results give plausible explanations for the large variation of σ_b reported in the literature and highlight the importance of selecting an appropriate TiO₂ reagent when targeting controllable σ_b in NBT-based ceramics. For ion-conducting applications (such as in intermediate-temperature solid oxide fuel cells, IT-SOFCs), rutile TiO₂ should be used, and for dielectric applications (such as in multilayer ceramic capacitors, MLCC) anatase TiO₂ should be used.