First-principles study on the multiferroic BiFeO3 (0001) polar surfaces

We present first-principles DFT+U calculations to investigate the stoichiometric (0001) polar surfaces of multiferroic R3c BiFeO3. We predict that the complete Fe-O-3-Bi trilayer, which is characterized by almost vanishing compensating charge, forms the most stable negative and positive surfaces. A large inward relaxation is found for the outermost Fe atomic layer at the negative surface, while the 03 atomic layer in the positive surface exhibits a remarkable in-plane rotational reconstruction. Our results show that the insulating nature of BiFeO3 persists at both surfaces but the negative termination is distinguished from the positive surface by the gap state. It is also found that the ferroelectric polarization and weak ferromagnetism of both surfaces have somewhat enhanced character because of relaxation and rehybridization of the surface atoms. Not only the different atomic/electronic structures but also the opposite polarization orientations indicate the distinct chemical properties between the negative and the positive (0001) surfaces and consequently many intriguing applications. (C) 2016 Elsevier B.V. All rights reserved.