Correlation between displacive-type ferroelectricity and electronic density of states near the Fermi level in SrBi2Ta2O9

The structural deformations around Sr, Bi, and Ta ions, and the electronic density of states near the Fermi level have been studied in paraelectric and ferroelectric phases of the layer pseudo‐perovskite compound, SrBi2Ta2O9 (SBT), by means of X‐ray absorption spectra (XAS) near the absorption edges of Sr, Bi, and Ta ions. The X‐ray absorption near edge structure (XANES) shows that localized largely structural deformations appear around Sr and Ta ions, but not around Bi ion in the vicinity of the ferroelectric to paraelectric structural phase transition. In both phases, there are the charge‐transfer (4dL, 4d2L2, and 4d3L3) bands of Sr, the negative charge‐transfer (5d9L′, 5d8L′2, and 5d7L′3) bands of Bi, and the charge‐transfer 5dnLm bands of Ta. The notations L and L′ represent a hole in the O‐2p valence band, and in the fully occupied 5d band of the Bi ions, respectively. The Sr‐4d, Bi‐5d, and Ta‐5d orbitals are largely hybridized with the 2p orbitals of the nearest‐neighbor oxygen ions occupying the (Bi2O2)2+ sheets or the pseudo‐perovskite blocks (SrTa2O7)2–. This hybridization is induced by an electronic interaction between the cations (Sr, Bi, and Ta ions) and the anions (O ions). The Fermi level is very close to the unoccupied Sr‐5pπ and Ta‐5d(eg) bands, but not the unoccupied Bi‐6dπ band. In the ferroelectric phase, the hybridization between the Bi ion and the oxygen ions (O2) from the (SrTa2O7)2– blocks considerably reduced and almost disappears. This reduction and disappearance of the hybridization increases the effective charges on O2 in the TaO6 octahedra, while the (Bi2O2)2+ layers act only as insulators. Thus, the charge‐up of O2 contributes a large dielectric permittivity of the pseudo‐perovskite (SrTa2O7)2– blocks, in addition to the structural deformation induced near the phase transition at 300 °C. The strong hybridization between the Sr and oxygen ions (O1, O4, and O5) also plays a crucial role to characterize the unusual ferroelectricity in SrBi2Ta2O9 family of compounds substituted Sr ions with other ions or vacancies. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)