High-temperature phase transitions in SrBi_2Ta_2O_9 film: a study by THz spectroscopy

TL;DR

This study uses THz spectroscopy to investigate high-temperature phase transitions in SrBi₂Ta₂O₉ films, revealing phonon behavior, relaxational modes, and symmetry changes across phases.

Contribution

It provides new insights into the dielectric and structural dynamics of SrBi₂Ta₂O₉ during phase transitions using THz spectroscopy.

Findings

The lowest frequency optic phonon decreases monotonically with temperature.

Dielectric anomaly explained by slowing relaxational mode.

Second harmonic generation confirms symmetry loss in ferroelectric phase.

Abstract

Time-domain THz transmission experiment was performed on a $\rm SrBi_2Ta_2O_9$ film deposited on sapphire substrate. Temperatures between 300 and 923 K were investigated and complex permittivity spectra of the film were determined. The lowest frequency optic phonon near 28 cm$^{-1}$ reveals a slow monotonic decrease in frequency on heating with no significant anomaly near the phase transitions. We show that the dielectric anomaly near the ferroelectric phase transition can be explained by slowing down of a relaxational mode, observed in the THz spectra. A second harmonic generation signal observed in a single crystal confirms a loss of center of symmetry in the ferroelectric phase and a presence of polar clusters in the intermediate ferroelastic phase.