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

Abstract

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, SrBi 2 Ta 2 O 9 (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, 4d 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(e g ) 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 (SrTa 2 O 7 ) 2-blocks considerably reduced and almost disappears. This reduction and disappearance of the hybridization increases the effective charges on O2 in the TaO 6 octahedra, while the (Bi 2 O 2 ) 2+ layers act only as insulators. Thus, the charge-up of O2 contributes a large dielectric permittivity of the pseudo-perovskite (SrTa 2 O 7 ) 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 SrBi 2 Ta 2 O 9 family of compounds substituted Sr ions with other ions or vacancies.

Metal–insulator transition induced by electronic and structural modulations in oxygen‐deficient perovskite‐type TbBaCo2O5.5

“…2, we deduce that the three pre-edge peaks observed around -27 eV, -20 eV and -15 eV at RT are 5dL, 5d A free Co 3+ ion has 3d 6 orbitals. In general, the degenerated 3d(t 2g ) and 3d(e g ) orbitals of the Co ions in the CoO 6 octahedra and CoO 5 pyramids are translated to a valence band and a conduction band due to the electronic configuration interaction [23,24]. For the electronic transition between two different orbitals, a 1s to 3d transition is a forbidden process, while a 1s to 2p transition is a permitted process.…”

Correlation between the metal–insulator transition and the electronic density of states near the Fermi level in oxygen‐deficient perovskite‐type NdBaCo2O5.5

“…This suggests that there are slightly cooperative displacements of the Ba ions only in the BaO networks for the M-I transition, like those in BaTiO 3 [22] and SrBi 2 Ta 2 O 9 [23]. Thus, we suspect that the Ba displacements made by the M-I transition affects directly the Ba-4d n L′ m bands, which are induced by the hybridization between the fully occupied Ba-4d orbitals and its surrounding O-2p orbitals in the BaO and CoO networks, and indirectly the Nd -4d…”

“…Thus, as described later, we suspected that the fine structures in the XANES and EXAFS should be intrinsic. Figure 2 and its inset show the XAS near the Nd-L III edge in the energy region of -40 eV to -10 eV and of -50 eV to 150 eV respectively, as those in SrBi 2 Ta 2 O 9 [23], taken from room temperature (RT) to 390 K. In the figures, the ordinate (I/I 0 ) is a count ratio between the incident X-ray photons (I 0 ) and the fluorescence X-ray photons (I) emitted from the sample, while the abscissa is the energy difference (E) from E 0 (Nd). The XAS curves in Fig.…”