Large electromechanical response in ferroelectrics: Beyond the morphotropic phase boundary paradigm

Ferroelectric based piezoceramics exhibiting large electromechanical response are used as sensors, actuators, and transducers in wide-ranging applications spanning sectors like space, defense, medical diagnostics, etc. In general, the large piezoelectric response in ferroelectric solid solutions is associated with a composition driven interferroelectric instability, commonly known as a morphotropic phase boundary (MPB). Here, we show that MPB is not necessarily required to achieve electromechanical response equivalent to, or even more than what can be achieved in MPB based ferroelectric solid solutions. We show this on two ferroelectric solid solution systems, namely, (1-x)PbTiO3-(x)Bi(Ni1/2Hf1/2 )O-3 (PT-BNH) and (Bi, La)FeO3 -PbTiO3 (BF-PT:La) which show large piezoelectric response (d(33) similar to 450 pC/N) and extraordinarily high electrostrain of similar to 1.3%, respectively. Although analogous to the conventional MPB systems, the critical compositions of these two alloys mimic a two-phase structural state (cubic + tetragonal), detailed analysis that suggests that it is not so. The cubic phase is rather a manifestation of short correlation length of the tetragonal regions and appears when the system is compositionally driven from a normal ferroelectric state to a relaxor ferroelectric state. This proves that, in contrast to conventional MPB systems, the large electromechanical response of the critical compositions of PT-BNH and BF-PT:La is not due to interferroelectric instability enabled polarization rotation. In the absence of the MPB, the sole contributor to large electromechanical response is a process associated with domain wall motion, large local polarization, and (non-MPB) lattice softening. The generalized ideas derived from our investigation offer scope for expanding the basket of high-performance piezoelectric materials by exploring solid solutions outside of the MPB framework