  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.
