Origin of high piezoelectricity at the morphotropic phase boundary (MPB) in (Pb0.94Sr0.06)(ZrxTi1-x)O3

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.