New multiferroic BiFeO₃ with large polarization

BiFeO₃ is one of the most widely studied multiferroic materials, because of its large spontaneous polarization at room temperature, as well as ferroelasticity and antiferromagnetism. Using an ab initio evolutionary algorithm, we found two new dynamically stable BiFeO₃ structures (P6₃ and P6₃22) at ambient pressure. Their energy is only 0.0662 and 0.0659 eV per atom higher than the famous R3c-BiFeO₃, and they have large spontaneous polarization, i.e., 71.82 μC cm^-2 and 86.06 μC cm^-2, respectively. The spontaneous polarization is caused by the movement of the Bi³⁺ atom along the [001] direction and mainly comes from the 6s electron of Bi³⁺. Interestingly, there is no lone pair electron of Bi³⁺, which is different from R3c-BiFeO₃. The new structures have the same magnetic configurations as R3c-BiFeO₃ (G-type antiferromagnetism), but they are characterized by one-dimensional channels linked by a group of two via surface-sharing oxygen octahedra. Due to the similarity of the two structures, both of them have indirect bandgap structures, and the bandgaps are 2.62 eV and 2.60 eV, respectively. This work not only broadens the structural diversity of BiFeO₃ but also has constructive significance for the study of spontaneous polarization of new structures of multiferroic materials.