A soft mode have been observed in the Raman spectra of SrBi2Nb2O9. The
frequency of the soft mode decreases and the soft mode becomes overda
mped on approaching the Curie point. Its linewidth diverges as GAMMA (
T) = GAMMA (0)[(T(c)-T)/T(c)]-1. The soft mode seems to be associated
with the ferro- to para- electric phase transition.