Fig. 5.

Radial quadrant domain structures in epitaxial BFO nanoplates subject to strain relaxation. a Schematic of a BFO rectangular nanoplate and an exotic quadrant domain structure therein. Red and blue lines represent the ferroelastic and ferroelectric domain walls, respectively. b Four representative buffer domain configurations stabilised by phase field simulation. c Schematics of the four types of domain configurations observed in the as-grown state. When one configuration can be superimposed on another configuration by symmetry operations of the point group 4 m, i.e., fourfold rotations, horizontal or vertical mirrors, their composite operations, both configurations are classified into the same type. Black arrows represent IP ferroelectric polarisations. The 180° ferroelectric domain walls are expressed by blue lines that form a semi-transparent gradient to represent the domain wall chirality (e.g., a clockwise Néel wall, as illustrated on the top). The expected vortex and antivortex points are marked on the schematics