Theory of ferroelectricity in Bi‐based multiferroics

Abstract

AbstractAn electron‐phonon (vibronic) theory of the Bi‐based multiferroics is developed. It is shown that the electron‐lattice covalency hybridization between the Bi 6s2 lone electron pair and empty oxygen 2p states causes instability of R25 and R15 polar vibrations in the reference cubic phase. The cubic perovskite structure is distorted by displacements of oxygen ions corresponding to the condensation of the R25 mode at the boundary point R = π /a (1, 1, 1) of the Brillouin zone (a is the cell constant of the cubic phase). These oxygen displacements correspond to the rotation of the FeO6 octahedron in BiFeO3 about the cube space diagonal. The triply degenerate mode Γ1u participates in the s‐p mixing of Bi lone electron pair states and 2p oxygen states. The ferroelectric instability holds in the R $ \bar 3 $c phase and the observed ferroelectric transition R $ \bar 3 $c → R 3c is connected with the softening of the A2u mode. The Γ1u mode in the R $ \bar 3 $c phase splits in Eu and A2u modes. We have calculated the spontaneous polarization Ps = 30–40 μC/cm2 (Ps = 1.5 μC/cm2 is also possible), Tc = 1105 K and dielectric constants. It is shown that the phase transition of the first order is in agreement with the experiment. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)