A new mechanism leading to a switchable polarization in a ferroelectric domain wall (DW) is proposed. A biquadratic coupling of the primary order parameter and its gradient triggers the phase transition in the DW with softening of the local polar mode and anomalous increase of the susceptibility at the phase transition temperature TDW. This mechanism describes the origin and properties of the polar Bloch and antipolar N\'eel components in the 180∘-DW of PbTiO3, which were recently reported from first-principles calculations.
@article{arxiv.2210.00793,
title = {Polar phase transition in $180^{\circ}$-domain wall of lead titanate},
author = {I. Rychetsky and W. Schranz and A. Tröster},
journal= {arXiv preprint arXiv:2210.00793},
year = {2022}
}