The ferroelectric soft mode and central mode in SrBi₂Ta₂O₉films

Abstract

The dynamics of the ferroelectric phase transition in a SrBi 2 Ta 2 O 9 film of 5.5 µm thickness deposited on sapphire was studied by means of timedomain terahertz transmission spectroscopy (30-300 K) and Fourier transform far-infrared transmission spectroscopy (300-950 K). The optical soft mode near 28 cm −1 (at 300 K) exhibits a weak monotonic softening down to 20 cm −1 at 950 K with no significant anomaly near either the ferroelectric or ferroelastic phase transition. An additional relaxation process below the phonon frequencies was resolved in the terahertz transmission spectra. The critical slowing down of its relaxation frequency could be responsible for the dielectric anomaly near the ferroelectric transition temperature. This indicates that the ferroelectric transition is predominantly of order-disorder type.

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

“…figure 1 in Ref. [15] and figure 1 in this article) just due to the increasing concentration of scratches in our film from tweezers (confirmed in microscope). Sample age had no influence on phonon frequencies.…”

“…Sample age had no influence on phonon frequencies. Figure 3 shows temperature dependence of ω 1 frequency obtained from TDTS and FIR transmission [15] spectra, the same values and temperature dependences were observed. FIR transmission spectra were fitted without CM, therefore also its dielectric strength ∆ε 1 is higher than ∆ε 1 obtained from TDTS with CM.…”

“…Oscillator fits of THz spectra without CM give the same ∆ε 1 parameters like the fit in Ref. [15], so different values of ∆ε 1 in figure 3 are not due to the different experimental methods but due to the different fitting procedure. Let us stress that the lowest frequency phonon is not the optic soft mode responsible for the ferroelectric PT, because it exhibits no anomaly in ω 1 frequency near either T c1 or T c2 .…”

“…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.…”

Dynamics of the phase transitions in Bi-layered ferroelectrics with Aurivillius structure: Dielectric response in the terahertz spectral range

“…10,[21][22][23] Surprisingly, the soft mode with frequency near 28 cm −1 has shown no anomaly around T c . Only a weak gradual softening of this mode was observed in the THz spectra during heating up to 950 K and, consequently, its contribution to the low frequency permittivity is much smaller than that needed to account for the reported dielectric anomaly near T c = 600 K. An additional relaxation was identified in the THz spectra and it was shown that its dielectric contribution is presumably responsible for the dielectric anomaly near T c .…”