*2.3.4. Fatigue effects in relaxors*

Fatigue effects in relaxor materials can be different from those of ferroelectrics in terms of phenomenology and microscopic origin.[23] Additionally, non-ergodic and ergodic relaxors have shown different electrical fatigue characteristics. In non-ergodic relaxors, a long-range ferroelectric order is stabilized after the application of a sufficiently large electric field and polarization reversal occurs during further electric field cycles. Ergodic relaxors undergo a weakly polar-to-polar state transition during electrical loading and they return into the weakly polar state during electric field unloading. This behavior has been widely observed in several bismuth-based perovskites.[24-27] The 0.94(Bi0.5Na0.5)TiO3-0.06BaTiO3 (BNT-BT) system can be classified as a non-ergodic relaxor. An addition of K0.5Na0.5TiO3 to BNT-BT determines a crossover to an ergodic relaxor behaviour, which can be observed also at room temperature. The non-ergodic relaxor BNT-BT has shown domain fragmentation during fatigue [28], while the ergodic compositions exhibit significantly higher fatigue resistance [27]. The current understanding is that the domain wall pinning effects become less significant in ergodic relaxor phases.[23] In addition, it can be considered that the ergodic relaxors return to a weakly polar state with low remanent polarization and low remanent strain during electric field unloading. This yields smoother variations of polarization and strain during cycling, which could probably be one of the reasons of the less pronounced fatigue effects. However, further studies are needed to better elucidate the mechanisms of the increased fatigue resistance in lead-free ergodic relaxors.
