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

Abstract

Polarized microscope observation of ferroelastic domains in a SrBi2Ta2O9 (SBT) single crystal reveals the presence of domains up to Tc1≃770 K, which supports the ferroelasticity and Amam symmetry of the intermediate phase between ferroelectric and paraelectric phases. Far-infrared spectra of SBT ceramics, single crystal and thin films show a well underdamped optical soft mode at 28 cm−1, which partially softens to 21 cm−1 near the ferroelectric transition temperature (Tc2≅600 K). This softening does not explain the entire anomaly of low-frequency permittivity observed near Tc2. On the basis of high-frequency measurements, which do not show a significant dispersion, central-mode-type dispersion in the 10–100 GHz range is proposed as an explanation. So, the phase transition at Tc2 apparently shows a crossover behavior between the displacive and the order–disorder type.

Temperature-driven phase transitions in SrBi2Ta2O9 from first-principles calculations

“…Structural studies in polycrystalline samples showed the existence of the intermediate phase (occurring between ~ 350 and 550 o C), although its space group (Fmmm [8], Amam [9,10] or B2cb [11]) was under debate. A direct observation of ferroelastic domains which disappears about 550 o C supports the ferroelasticity and the Amam symmetry of the intermediate phase [12]. Surprisingly, SrBi 2 Nb 2 O 9 shows a single crystallographic phase transition [13] which seem to exhibit a direct phase transition from the high-temperature tetragonal to a ferroelectric phase.…”

“…This is in agreement with the results of Kamba et.al. who observed a gradual disappearance of elastic domains on heating, suggesting that the transition at T c2 is near second order [12].…”

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

“…Such temperature behaviour cannot explain dielectric anomaly observed in low-frequency dielectric data near T c1 , therefore the name soft mode is not correct. 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].…”

“…S Second harmonic generation (SHG) signal was measured on SBT single crystal with dimensions of about 2 mm x 1mm x 0.1 mm, its large face was perpendicular to the c axis. The same crystal was used in a previous studies [9,14]. Q-switched Nd-YAG laser served as a light source.…”

“…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.…”