Formation of monoclinic nanodomains at the morphotropic phase boundary of ferroelectric systems

The monoclinic (M) phase reported recently in ferroelectric solid solutions having compositions at the morphotropic phase boundary (MPB) between the tetragonal and rhombohedral phases could be responsible for the extraordinary piezoelectric properties of these materials, but its very existence is under serious debate. In this article we show by computer simulation that using a sixth order Landau free energy polynomial with small polarization anisotropy at the MPB and taking into account the electrostatic and elastic interactions, a hierarchical structure consisting of M phase nanodomains is formed within a composition or temperature range enclosing the MPB. It is found that the M phase, even though unstable according to the Landau free energy, is stabilized by the long-range elastic and electrostatic interactions. A new phase diagram containing the M phase is established based on the simulation results.