Extrinsic Rayleigh coefficient and domain mobility in critical MPB compositions of high-performance piezoceramics: A revisit

Enhanced domain mobility is considered as one of the important characteristics of morphotropic phase boundary (MPB) ferroelectrics exhibiting a very large piezoelectric response. For nearly two decades, Rayleigh analysis of dielectric and piezoelectric response at sub-coercive fields has been used as a phenomenological tool to determine the relative contribution of domain walls in influencing polar properties in these systems. The high value of the extrinsic Rayleigh coefficient at the MPB is generally attributed to enhanced domain mobility. Using coercive field as a measure of hindrance to domain mobility, in this paper, we have examined the correlation between the extrinsic Rayleigh coefficient, piezoelectric response, and coercive field (EC) by a comparative study on three different high performance piezoelectrics (i) (BaTi0.88Sn0.12)-(Ba0.7Ca0.3)TiO3, (ii) Pb(Zr,Ti)O3 and (iii) PbTiO3-BiScO3 across their respective MPBs. Our study shows that although the three systems exhibit maximum piezoelectric response and maximum extrinsic Rayleigh coefficient for their respective MPB compositions, the domain mobility in the MPB compositions of the Pb-based systems is not the highest. The lead-free system, on the other hand, exhibits highest domain mobility for the critical MPB composition. Contrary to the common perception, our study reveals that large value of the Rayleigh coefficient at the MPB is not necessarily indicative of enhanced domain mobility. We argue that the large piezoresponse in MPB systems is a result of complex interplay of field induced motion of the interphase boundaries and domain walls. In the lead-based systems the interphase boundary motion hinders domain wall mobility. The large value of the Rayleigh coefficient and piezoresponse in the Pb-based systems is because of the dominant role of the interphase boundary motion instead of the enhanced domain wall mobility.