Mixed Bloch-Néel-Ising character of 180° ferroelectric domain walls

Ferroelectric $180\ifmmode^\circ\else\textdegree\fi{}$ domain walls are well-known to be predominantly Ising-like. Using density functional theory, and molecular dynamics simulations, the $180\ifmmode^\circ\else\textdegree\fi{}$ domain walls in prototypical ferroelectrics lead titanate $({\text{PbTiO}}_{3})$ and lithium niobate $({\text{LiNbO}}_{3})$ are shown to have mixed character; while predominantly Ising-like, they also manifest some Bloch- and N\'eel-like character. Phase-field calculations show that such mixed wall character can be dramatically enhanced in nanoscale thin film heterostructures such as ${\text{BaTiO}}_{3}/{\text{SrTiO}}_{3}$, where the internal wall structure can form polarization vortices. Such mixed character walls can be expected to exhibit dynamical wall properties distinct from pure Ising walls.