Polarized microscope observation of ferroelastic domains in a SrBi2Ta2O9 (SBT) single crystal reveals the presence of domains up to T(c1)similar or equal to770 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 (T(c2)congruent to600 K). This softening does not explain the entire anomaly of low-frequency permittivity observed near T-c2. 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 T-c2 apparently shows a crossover behavior between the displacive and the order-disorder type. (C) 2002 American Institute of Physics.