  There is a general perception that large piezoelectric response in
ferroelectric alloys requires tuning the system towards a morphotropic phase
boundary (MPB), i.e., a composition driven inter-ferroelectric instability.
Here we show that high piezoelectric response can be realized even in non-MPB
alloy systems. This is demonstrated on (1-x)PbTiO3-(x)Bi(Zr0.5Ni0.5)O3 (PT-BNZ)
by a comprehensive study involving electric-field and temperature dependent
XRD, Raman spectroscopy, dielectric, piezoelectric and high field electrostrain
measurements. We found that poling-field irreversibly suppresses the cubic-like
phase at room temperature. Based on our results, we argue that that which
appears as MPB, comprising of tetragonal and cubic-like phases on the global
scale, is not so actually. The large piezoresponse is due to coexistence of
tetragonal regions of long and short-range coherence. The PT-BNZ system is
therefore qualitatively different from the conventional MPB systems such as
PZT, PMN-PT, and PbTiO3-BiScO3, etc., which exhibits coexisting tetragonal and
rhombohedral/monoclinic phases in thermodynamic equilibrium. In the absence of
inter-ferroelectric instability as a phenomenon, field induced
polarization-rotation and inter-ferroelectric transformation are no longer
plausible mechanisms to explain the large piezoelectric response in PT-BNZ. The
large piezoelectricity is primarily due to enhanced mobility of the tetragonal
domain walls enabled by domain miniaturization. Our study proves that
attainment of large piezoelectricity does not require inter-ferroelectric
instability as a necessary criterion.
