Density functional investigations on electronic structures, magnetic ordering and ferroelectric phase transition in multiferroic Bi2NiMnO6

Using the full-potential linearised augmented-plane wave (FP-LAPW) method based on density functional theory (DFT), we have investigated the electronic structures, the magnetic behavior, and the ferroelectric origin of multiferroic Bi2NiMnO6. The calculated ferromagnetic Curie temperature of Bi2NiMnO6 is very sensitive to the Mn4+—O2-—Ni2+ length. When average Mn4+—O2-—Ni2+ length increases from 3.82 to 4.05 Å, the Curie temperature increases from 179 to 295 K. The Mn4+—O2-—Ni2+ superexchange interaction due to the virtual hopping of electrons from O-2p filled states to Mn-/Ni-3d empty states is enhanced when the band gap formed by crystal-field splitting decreases, thus the effective exchange parameters and Curie temperature increase as Mn4+—O2-—Ni2+ length increases. The ferroelectric distortion in Bi2NiMnO6 is directly from the hybridization of Bi-6p and O-2p states. The role of Bi-6s2 lone pairs electrons may be that hybridized O-2p with Bi-6s orbitals may be more appropriate in compatible symmetry with Bi-6p orbital than O-2p orbital only. Furthermore, the route of ferroelectric distortion in Bi2NiMnO6 from paraelectric P21/n phase to ferroelectric C2 phase is discussed