FERROELASTIC PHASE-TRANSITION IN BISMUTH TITANATE

The Raman spectra of Bi2Ti4O11 at hydrostatic pressure or high tempera ture show that the frequency of the soft mode has a non-zero value at the phase transition point. The Raman spectra of Bi1.8La0.2Ti4O11 and Bi1.8Yb0.2Ti4O11 at high pressure reveal that there is an additional p honon mode in the low-frequency region. The finite soft mode frequency has proved to arise from the coupling between the soft mode and the a dditional phonon mode which may be a zone boundary mode. The phase tra nsition can be considered as a second-order ferroelastic one driven by the soft mode.