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 ${T}_DW<{T}_c$. This mechanism describes the origin and properties of the polar Bloch component, which appears below ${T}_DW$ additionally to the antipolar N\'eel component in the ${180}^{\ensuremath{\circ}}$ DW of ${\mathrm{PbTiO}}₃$. The tensile strain of the DW plane promotes the development of the Bloch polarization.