Atomic-scale polar vortices in Na0.5Bi0.5TiO3 grains

Ferroelectric materials have great potential applications in non-volatile ferroelectric random access memories because of the singularity of one-dimensional topological defects. In this paper, we have found for the first time the atomic-level dipole rotations of vortex structures in crystalline materials. By the aberration-corrected high resolution scanning transmission electron microscopy (STEM), the atomic-scale multiple rotation domains and polar vortices in Na0.5Bi0.5TiO3 (NBT) grains have been clearly observed in lead-free perovskite NBT. Meanwhile, nanobeam electron diffraction experiments provided the strong evidences of the coexistence of tetragonal, orthorhombic and rhombohedra phases at room temperature. These polar vortices are constructed by the competition of these atomic-scale polar domains, which are originated from the multiphase coexistence of NBT and can be well confirmed by our theoretical calculations. These observations have put forward new implications for the creation of vortex topologies in crystalline materials without sophisticated thin-film technologies, and it can also help to understand the complicated polar mechanism in lead-free perovskite ferroelectric materials.