High oxide ion conductivity in layer-structured Bi₄Ti₃O₁₂-based ferroelectric ceramics

The layer-structured perovskite Bi₄Ti₃O₁₂ is a well-known lead-free ferroelectric and piezoelectric material with potential applications in both non-volatile memory devices and high-temperature sensors. In this paper, high oxide ion conductivity is observed for modified Bi₄Ti₃O₁₂-based ceramics, and their associated oxygen transport mechanism is investigated systemically. Impedance data and dc conductivity as a function of oxygen partial pressure reveal that Bi₄Ti₃O₁₂ shows mixed oxygen ion and hole conduction. Bi deficiency in the starting composition leads to no change in the conduction mechanism and no significant increase in bulk conductivity (σ_b). Interestingly, high oxide ion conductivity is achieved with further Mg²⁺ acceptor doping at the B site. The optimum composition with a nominal formula of Bi_3.88Ti_2.92Mg_0.08O_11.74 exhibits a high σ_b of ∼10^-2 S cm^-1 and a high ionic transport number (t_ion) of ∼0.92 at 650 °C, which are well comparable to those of leading solid oxide electrolyte materials. Furthermore, this composition shows extremely low conductivity degradation at 450 °C, 600 °C and 700 °C. The present work opens up new horizons for Bi₄Ti₃O₁₂ as an oxide ion conductor in addition to ferroelectric- and piezoelectric-based applications.