Oxide Ion Conductors Developed from the Layer-Structured Ferroelectric Perovskite Bi <sub>4</sub>Ti <sub>3</sub>O <sub>12</sub>

Abstract

Layer-structured perovskite Bi4Ti3O12 is a well-known lead-free ferroelectric and piezoelectric material. By solving the problems of high leakage current and polarization degradation associated with oxygen vacancies, Bi4Ti3O12 shows potential applications in both non-volatile memory devices and high-temperature sensors. In this paper, the high leakage is transformed into high level of ionic conductivity and new oxide ion conductors are developed from Bi4Ti3O12, and the oxygen transport mechanism in the layer-structured perovskite is investigated. Different Bi-stoichiometric compositions are presumably mixed hole and ion conductors with bulk conductivity (σb) of 10-4 and 10-3 S/cm orders of magnitude at 600 °C and 700°C, respectively. It is interesting that a combination of Bi deficiency and Mg doping on the Ti site results in high-performance oxide ion conduction for Bi4Ti3O12. Nominal starting composition with a formula of Bi3.88Ti2.92Mg0.08O11.74 exhibits oxide ion conductivity with σb of ∼10-2 S/cm and ionic transport number (tion) of 0.92 at 650 °C, which are well comparable to those of other known solid oxide electrolyte materials. Furthermore, this composition shows low conductivity degradation and good chemical stability in reducing atmosphere (5%H2/95%N2). This study opens up new horizons for Bi4Ti3O12 and other A-site Bi-based layer-structured perovskites as oxide ion conductors in addition to ferroelectric- and piezoelectric-based applications.