Abstract
ABSTRACT 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). The ferroelectric instability holds in the phase and the observed ferroelectric transition is connected with the softening of the A2u mode. The Γ1u mode in the 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.
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