The possible existence of a second phase transition in Aurivillius-structure-type ferroelectric compounds is discussed. For this purpose, measurements of the complex elastic (Young's) modulus, dielectric constant, DC conductivity and Raman spectra as a function of temperature have been performed. The results suggest that the anomalies that appear in macroscopic complex elastic modulus, DC conductivity and, probably, those of the 25 cm(-1) vibration mode of Raman spectra near the temperature of 300 degrees C do not correspond to a ferro-paraelectric or ferro-ferroelectric phase transition. Anomalies in the dielectric constant have only been found at the phase transition temperatures. Brillouin measurements show that the 300 degrees C anomaly is of extrinsic character, which could be connected to shallow point defects caused by the Bi3+ instability in the Bi2O2 layer. With increasing temperature, the defects are ionized increasing the DC conductivity and changing the elastic properties of the material by pinning domain walls or dislocations. Electron spin resonance (ESR) spectra, obtained after heating the samples at T = 500 degrees C, show the line of the almost free electron from the ionized defects.