Differences in nature of electrical conductions among Bi₄Ti₃O₁₂-based ferroelectric polycrystalline ceramics

Bismuth titanate Bi₄Ti₃O₁₂ (BiT), was one of the most promising lead-free high-temperature piezoelectric materials, due to high Curie temperature (675 °C) and large spontaneous polarization (50 μC/cm²); however, extensive studies had revealed that high leakage conductivity interferes with the poling process, hindering its practical applications. In this paper, an electrically insulating property was achieved by a low level Nb donor substitution to suppress a high level of holes associated with high oxygen vacancy concentration. Bi₄Ti_2.97Nb_0.03O₁₂ ceramic showed significant enhancements of electrical resistivity by more than three order of magnitude and activity energy with value >1.2 eV, which are significant for piezoelectric applications of BiT-based materials. However, pure and A₂O₃-excess (A = Bi, La and Nd; 3 at %) BiT ceramics, were mixed hole and oxygen ion conductors. Schottky barriers were both formed at grain boundary region and the sample-electrode interface, because of the existence of semiconducting bulk. Interestingly, the electron conduction could be suppressed in N₂, as a consequence, they became oxide ion conductors with conductivity of about 4 × 10^-4 S cm^-1 at 600 °C.