Evidence against the polarization rotation model of piezoelectric perovskites at the morphotropic phase boundary

The origin of the very large piezoelectric response observed in the vicinity of the morphotropic phase boundary (MPB) in perovskite lead zirconate titanate and related systems has been under intensive studies. Polarization rotation ideas are frequently invoked to explain the piezoelectric properties. It was recently reported that lead titanate undergoes a phase transformation sequence $P4mm\to Pm\to Cm\to R\bar{3}c$ at 10 K as a function of hydrostatic pressure [M. Ahart et al. Nature Letters. \textbf{451}, 545 (2008)]. We demonstrate that this interpretation is not correct by (i) simulating the reported diffraction patterns, and (ii) by density-functional theory computations which show that neither the $Pm$, $Cm$ nor $Pmm2$ phase is stable in the studied pressure range, and further show that octahedral tilting is the key stabilization mechanism under high pressure. Notes on a more general ground are given to demonstrate that a continuous phase transition between rhombohedral and tetragonal phases via intermediate monoclinic phase is not possible. Thus, two-phase co-existence in the vicinity of the phase transition region is probable and has an important role for electromechanical properties.