  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.
