Impedance spectroscopy of SrBi₂Ta₂O₉ and SrBi₂Nb₂O₉ ceramics correlation with fatigue behavior

In this research, a fatigue model for ferroelectric materials is proposed. The reasons for the electrical fatigue resistance of SrBi₂Ta₂O₉ (SBT), SrBi₂Nb₂O₉ (SBN), and PbZr_1−xTi_xTi_xO₃ (PZT) are discussed in terms of the bulk ionic conductivities of the compounds. To obtain the bulk ionic conductivity of SBT and SBN, we have used impedance spectroscopy which provides an effective method that allows us to separate the individual contributions of bulk, grain boundaries, and electrode-ferroelectric interfaces from the total capacitor impedance. The bulk ionic conductivities of SBT and SBN (∼10⁻⁷ S/cm) are much higher than those of the perovskite ferroelectrics, e.g., PZT (∼10⁻¹¹–10⁻¹⁰ S/cm). The high ionic conductivities led us to conclude that the good fatigue resistance of SrBi₂Ta₂O₉ and SrBi₂Nb₂O₉ is due to easy recovery of defects. Specifically, oxygen vacancies entrapped within the capacitors are easily released, resulting in limited space charge buildup and domain wall pinning during the polarization reversal process. However, the oxygen vacancies in PZT are trapped at trap sites to become space charges, resulting in capacitor fatigue.