High-temperature phase transitions in SrBi2Ta2O9 film: a study by THz spectroscopy

A time-domain THz transmission experiment was performed on a SrBi2Ta2O9 film deposited on a sapphire substrate. Temperatures between 300 and 923 K were investigated and complex permittivity spectra of the film were determined. The lowest-frequency optical 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 the slowing down of a relaxational mode, observed in the THz spectra. A second-harmonic generation signal observed for a single crystal confirms the loss of a centre of symmetry in the ferroelectric phase and the presence of polar clusters in the intermediate ferroelastic phase