Anomalous electrical performance of A-site double-bivalent-doped Bi0.49Na0.5TiO3-δ ceramics from nominal oxygen deficiency to excess

Abstract Perovskite structural Bi0.5Na0.5TiO3 (BNT) ferroelectrics can exhibit considerable ionic conductivity, giving a new area for the application as oxygen ion conductors. Through acceptor doping and A-site nonstoichiometry, an ionic conductive mechanism associated with well-resolved arcs in the complex impedance spectra is proposed. Different dielectric responses in ceramics can be deduced to probe the electrical inhomogeneities in separative regions, such as bulk, grain boundary, and electrode, regions. Generally, ionic conductivity only arises at the nominal oxygen deficiency composition of BNT based ceramics. Although large current leakage can exist in the BNT ferroelectrics, they overall display an insulative nature and dominate the electronic conductive contribution, and only a large main arc can be identified in the Nyquist plots. In this work, A-site bivalent doped Bi0.49-x(SrBa)xNa0.5TiO3-δ and Bi0.49-x(SrCa)xNa0.5TiO3-δ ceramics ranging from oxygen deficiency to excess are investigated. However, anomalous ionic conductive characteristics are achieved at nominal oxygen excess composition. Herein, the polarization and conductive mechanisms are discussed to elucidate the effect of compositions and their phase and microstructure on the AC impedance, dielectric, and ferroelectric performances. The inhomogeneous distribution of oxygen vacancies, resulting from element segregation or distorted phases in the respective electroactive domains, is a crucial issue in the design of BNT based dielectrics or ionic conductors.