**5. Summary and conclusions**

In this chapter, I have reviewed and discussed a case study on BFO ceramics explaining how the presence and dynamics of DWs showing enhanced electrical conductivity with respect to bulk conductivity can have a crucial effect on the macroscopic piezoelectric response of BFO. The mechanism goes far beyond the expected DW-defects pinning interactions, described in Section 2 of this chapter, and reflect itself in the nonlinear and hysteretic piezoelectric response of BFO. The unusual features of the response, i.e., a hard-to-soft transition induced by lowering the driving electric-field frequency and negative piezoelectric phase angle, can be explained by nonlinear piezoelectric Maxwell-Wagner effects. Simple analytical modeling confirms these macroscopic-response features and show that the key entities leading to such effects are conductive DWs. I could envision few points that can be drawn from the presented results. First, the data clearly show that local conductive paths (such as those along DWs) should be considered more seriously in addition to conventional bulk conductivity, which is mostly discussed in the literature on BFO. This is particularly important for the development of next-generation BFO-based piezoceramics for high-temperature applications as the local conductivity is what makes the response of BFO unstable in terms of driving field parameters (amplitude and frequency) and, most importantly, temperature. Second, the key problem related to M-W effects is that the response is boosted only at quasi-static driving conditions, as shown earlier by modeling of ceramic-polymer composites [45]. While certainly limited in the frequency range, it could be interesting to test anisotropic effects and piezoelectric enhancements in engineered matrices containing highly conducting charged DWs. It seems reasonable, though, that one should first validate the idea by modeling.
