Abstract
It is shown that the electron-lattice covalent 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 ferroelectric instability holds in the phase and the observed ferroelectric transition is connected with the softening of the A2u mode. We take into account vibronic couplings in the Bi-based multiferroics. We obtained the free energy of the Bi-based multiferroics taking into account the Zeeman splitting of electronic levels in an external magnetic field. On the base of the free energy we investigated the magnetoelectric effects in the Bi-based multiferroics.
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