Inherent nanoscale structural instabilities near morphotropic boundaries in ferroelectric solid solutions

Abstract

The special structural properties of ferroelectric solid solutions near morphotropic phase boundaries are explained on thermodynamic grounds. The authors show that vanishing of the orientational anisotropy in the simplest 2-4-6 Landau polynomial is sufficient to produce the appearance of a morphotropic boundary (MB) separating rhombohedral and tetragonal phases. Under this condition the polarization vector becomes unstable against rotation, resulting in a drastic decrease in domain wall energy and the formation of mixed nanodomains having a diffraction domain-averaged structure that may mimic diffraction from lower-symmetry monoclinic phases. The theory automatically predicts tricritical points on the line of paraelectric to ferroelectric transitions and continuity of thermodynamic properties across the MB.