Stability and electronic structure of BiFeO3 (111) polar surfaces by first-principle calculations

The structural and electronic properties of R3cR3c phase BiFeO3 (111) surface were calculated by the full-potential augmented plane wave method within the LSDA+ULSDA+U. It is found that the (111)-Bi/O3/Fe-terminated surface is more stable than the O- and Fe-terminated surfaces at the O-poor environment, and (111)-O3/Fe/Bi-terminated surface is more stable than the Bi- and Fe-terminated surfaces at the O-rich environment. Indeed, the (111)-Fe/Bi/O3-terminated surface cannot be stabilized, after relaxation it will turn to the Bi-terminated surface. The density of states (DOS) of the relaxed BiFeO3 (111) surface were calculated and compared with that of the bulk BiFeO3. The ground-state electronic property of ferroelectric BiFeO3 is found to be insulating and antiferromagnetic, but the BiFeO3 (111) surface all have metallic properties, and the Fe magnetic moments are coupled ferromagnetically (F) within the pseudocubic (111) planes and antiferromagnetically (AF) between adjacent planes.