On the existence of a second phase transition in ferroelectrics with Aurivillius-type structure through the study of the Young's modulus

The possible existence of a second phase transition in Aurivillius-structur e-type ferroelectric compounds is discussed. For this purpose, measurements of the complex elastic (Young's) modulus, dielectric constant, DC conducti vity and Raman spectra as a function of temperature have been performed. Th e results suggest that the anomalies that appear in macroscopic complex ela stic modulus, DC conductivity and, probably, those of the 25 cm(-1) vibrati on mode of Raman spectra near the temperature of 300 degrees C do not corre spond to a ferro-paraelectric or ferro-ferroelectric phase transition. Anom alies in the dielectric constant have only been found at the phase transiti on temperatures. Brillouin measurements show that the 300 degrees C anomaly is of extrinsic character, which could be connected to shallow point defec ts caused by the Bi3+ instability in the Bi2O2 layer. With increasing tempe rature, the defects are ionized increasing the DC conductivity and changing the elastic properties of the material by pinning domain walls or dislocat ions. Electron spin resonance (ESR) spectra, obtained after heating the sam ples at T = 500 degrees C, show the line of the almost free electron from t he ionized defects.