Domain walls in a perovskite oxide with two primary structural order parameters: First-principles study of BiFeO₃

We present a first-principles study of ferroelectric domain walls (FE-DWs) in multiferroic BiFeO₃ (BFO), a material in which the FE order parameter coexists with antiferrodistortive (AFD) modes involving rotations of the O₆ octahedra. We find that the energetics of the DWs are dominated by the capability of the domains to match their O₆ octahedra rotation patterns at the plane of the wall, so that the distortion of the oxygen groups is minimized. Our results thus indicate that, in essence, it is the discontinuity in the AFD order parameter, and not the change in the electric polarization, that decides which crystallographic planes are most likely to host BFO's FE-DWs. Such a result clearly suggests that the O ₆ rotational patterns play a primary role in the FE phase of this compound, in contrast with the usual (implicit) assumption that they are subordinated to the FE order parameter. Our calculations show that, for the most favorable cases in BFO, the DW energy amounts to several tens of mJ/m2, which is higher than what was computed for other ferroelectric perovskites with no O₆ rotations. Interestingly, we find that the structure of BFO at the most stable DWs resembles the atomic arrangements that are characteristic of low-lying (meta)stable phases of the material. Further, we argue that our results for the DWs of bulk BFO are related with the nanoscale-twinned structures that Prosandeev [Adv. Funct. Mater. (2012)] have recently predicted to occur in this compound, and suggest that BFO can be viewed as a polytypic material. Our work thus contributes to shape a coherent picture of the structural variants that BFO can present and the way in which they are related.