New multiferroic BiFeO with large polarization.

BiFeO 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 evolutionary algorithm, we found two new dynamically stable BiFeO structures (6 and 622) at ambient pressure. Their energy is only 0.0662 and 0.0659 eV per atom higher than the famous 3-BiFeO, and they have large spontaneous polarization, , 71.82 μC cm and 86.06 μC cm, respectively. The spontaneous polarization is caused by the movement of the Bi atom along the [001] direction and mainly comes from the 6s electron of Bi. Interestingly, there is no lone pair electron of Bi, which is different from 3-BiFeO. The new structures have the same magnetic configurations as 3-BiFeO (G-type antiferromagnetism), but they are characterized by one-dimensional channels linked by a group of two 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 BiFeO but also has constructive significance for the study of spontaneous polarization of new structures of multiferroic materials.