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 in perovskite lead zirconate titanate and related systems has been under intensive study. Polarization rotation ideas are frequently invoked to explain the piezoelectric properties. It was recently reported that lead titanate undergoes a phase transformation sequence P4mm -> Pm -> Cm -> R (3) over barc at 10 K as a function of hydrostatic pressure (Ahart et al 2008 Nature 451 545). We demonstrate that this interpretation is not correct (i) by simulating the reported diffraction patterns, and (ii) by means of density-functional theory computations which show that the Pm, Cm and Pmm2 phases are all unstable in the pressure range studied, and further show that octahedral tilting is the key stabilization mechanism under high pressure. Notes on more general grounds are given to demonstrate that a continuous phase transition between rhombohedral and tetragonal phases via an intermediate monoclinic phase is not possible. Thus, two-phase coexistence in the vicinity of the phase transition region is probable and has an important role as regards electromechanical properties.