  In this work, we address the issue of peaking of piezoelectric response at a
particular composition in the morphotropic phase boundary (MPB) region of
(Pb0.940Sr0.06)(ZrxTi1-x)O3 (PSZT) piezoelectric ceramics. We present results
of synchrotron x-ray powder diffraction, dielectric, piezoelectric and sound
velocity studies to critically examine the applicability of various models for
the peaking of physical properties. It is shown that the models based on the
concepts of phase coexistence, polarization rotation due to monoclinic
structure, tricritical point and temperature dependent softening of elastic
modulus may enhance the piezoelectric response in the MPB region in general but
cannot explain its peaking at a specific composition. Our results reveal that
the high value of piezoelectric response for the MPB compositions in PSZT at
x=0.530 is due to the softening of the elastic modulus as a function of
composition. The softening of elastic modulus facilitates the generation of
large piezoelectric strain and polarization on approaching the MPB composition
of x=0.530. This new finding based on the softening of elastic modulus may pave
the way forward for discovering/designing new lead-free environmentally
friendly piezoelectric materials and revolutionize the field of piezoelectric
ceramics.
