First-principles study on the electronic structure of bismuth transition-metal oxides

The electronic structure, magnetic and electric properties, and lattice stability of multiferroic BiMnO3 as a typical system in perovskite Bi transition-metal oxides (BiMO3) are studied from first principles. It is demonstrated theoretically for the first time that the orbital ordering within the Mn e(g) orbitals is actually realized in BiMnO3, being consistent with crystallographic data, and plays a crucial role in the appearance of ferromagnetism. Total-energy calculation shows the ferromagnetic state is indeed stabilized. Electrical polarization of BiMnO3 is also estimated based on the Berry phase theory. Lattice instability to off-centred displacement, which is driven by strong covalent bonding between Bi 6p and O 2p states, is found to be rather common in the BiMO3 series.