180 degree domain walls in single crystal BiFeO3

We report a study of 180 degree domains in flux-grown BiFeO3 single crystals using transmission electron microscopy. Previous studies (Berger et al. 2012, Phys. Rev. B 85 064104; and Jia et al. 2015, Acta Mat. 82 156) found a complex domain structure consisting of alternating sawtooth and planar structures with a period of 50-100 nm. Here, we show using a combination of diffraction contrast transmission electron microscopy (TEM), convergent beam electron diffraction and atomic resolution scanning TEM, that sawtooth walls are tail-to-tail 180 degree domain walls, alternating with flat head-to-head domain walls lying on (112) planes, lying at ~70 degrees to the [111] polar axis (in pseudo-cubic indexing). The flat head-to-head domain walls are coincident with a 1/2[111] stacking fault, also showing reconstruction visible in atomic resolution images (Fig. 1). The tail-to-tail domain walls form a crinkled 3D structure with three-faceted teeth consisting of a (11-2) facet allowing charge-free reversal of polarisation, and two facets close to (3-21) and (-321). Vortex structures are found at these domain walls where the chirality of the Ising-like change in polarisation reverses (Fig. 2).

Fig. 1 Flat head-to-head 180 degree domain walls in BiFeO3 (negative displacement of Fe relative to Bi shown).

Fig. 2 Sawtooth tail-to-tail 180 degree domain walls in BiFeO3 (positive displacement of Fe relative to Bi shown)