Landau-Ginzburg-Devonshire theory of the chiral phase transition in 180° domain walls of PbTiO₃

A new mechanism leading to a switchable Bloch-type polarization in a domain wall separating two ferroelectric domain states is proposed. A biquadratic coupling of the primary order parameter and its gradient originating
from inhomogeneous electrostriction triggers the chiral phase transition (Ising-to-Bloch) in the domain walls (DW) with softening of the local polar mode and anomalous increase of the dielectric susceptibility at the phase transition temperature TDW < Tc. This mechanism describes the origin and properties of the polar Bloch component, which appears below TDW additionally to the antipolar Néel component in the 180◦ DW of PbTiO₃.
The tensile strain of the DW plane promotes the development of the Bloch polarization.