Infrared activity in the Aurivillius layered ferroelectricSrBi2Ta2O

Abstract

Experimental studies were carried out on infrared-active phonons in the Aurivillius ferroelectric SrBi 2 Ta 2 O 9 ͑SBT͒, using reflectivity measurements ͑down to 200 cm Ϫ1 ͒ and transmission measurements ͑down to 100 cm Ϫ1 ͒ on crystals, pellets, and thin films. An analysis was done of the contrasting consequences of the competing orthorhombic and pseudotetragonal symmetries in SBT. Reflectivity results for light polarized in the ab plane show that the anisotropy in this plane is small in the frequency range from 300 to 1200 cm Ϫ1 , indicating the influence of the approximate tetragonal symmetry. Dielectric dispersion properties were derived for this polarization (EЌc) in this frequency range. The transverse-optical ͑TO͒ and longitudinal-optical ͑LO͒ frequencies corresponding to the dominant EЌc band are 613 and 773 cm Ϫ1 , respectively. This strong, highfrequency band arises from a mode dominated by the motion of the oxygen sublattice; its TO and LO frequencies yield an oxygen-ion Szigeti effective charge of Ϫ1.5e. Frequency estimates for the TO ͑LO͒ pairs of other strong bands are 188͑330͒ and 334(451) cm Ϫ1 for EЌc, and 610͑675͒ and 780(815) cm Ϫ1 for Eʈc. In addition to the main infrared bands, the main Raman bands of SBT are also reported.

Ferroelastic phase in SrBi2Ta2O9 and study of the ferroelectric phase-transition dynamics

“…The roomtemperature spectrum above 200 cm Ϫ1 agrees with that in Ref. 16. The reflection bands are best resolved in the lowtemperature spectra where the mode dampings are smaller.…”

“…The SM in the FE phase is expected to be active in both IR and Raman spectra. 16 Moret et al 16 reported IR reflectivity and transmission studies on FE SBT, but their work did not extend into the far-IR SM region. In the mid-IR, they found that the high-frequency ͑HF͒ modes in SBT showed the strong influence of the pseudotetragonal parent symmetry ͑e.g., approximate Raman IR mutual exclusion͒, but this will not extend to the symmetry-breaking SM.…”

“…Two types of SBT samples were investigated: Single crystal, as used in a previous study, 16 and ceramics. The single crystal, a plate with dimensions of about 2 mm ϫ2 mmϫ0.1 mm, was grown from the melt: 5 Its large face was perpendicular to the c axis.…”

High-temperature phase transitions in SrBi2Ta2O9film: a study by THz spectroscopy

“…We proposed existence of an additional relaxation mode, which will slow down to T c1 and will cause dielectric anomaly near T c1 [9]. This relaxation we really observed in recent THz spectra below 300 K [14,16]. The aim of this paper is the continuation of the previous work at higher temperatures and to bring THz dielectric spectra of SBT thick film (same like in Ref.…”

“…Ferroelectric structure has space group A2 1 am (Z prim =2) and undergoes the phase transition (PT) to paraelectric phase at T c1 near 600 K. Recently was discovered intermediate tetragonal ferroelastic phase with Amam space group(Z prim =2) [7,8,9] which undergous to paraelectric paraelastic phase with I4/mmm (Z prim =1) structure above T c2 ≅ 770 K. This second phase transition is improper ferroelastic (with doubling of the primitive unit-cell volume on cooling) and the lower temperature one is proper ferroelectric (no multiplication of the primitive unit-cell volume with respect to the ferroelastic phase) [9]. The dynamics of the PT's in SBT has been investigated first by Raman scattering [10,11,12], later by Brillouin scattering [13] and Fourier transform infrared (FTIR) reflectivity and transmission spectroscopy [9,14,15]. It was shown that the lowest frequency optic phonon near 28 cm -1 (at 300 K) reduces its frequency on heating and even disappears from Raman spectra above T c1 due to the change of selection rules.…”