Abstract
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 
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 Pmm 2
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.