HYPER-RAMAN AND RAMAN STUDIES ON THE PHASE-TRANSITION OF FERROELECTRIC LITAO(3)

Hyper-Raman and Raman spectra of ferroelectric LiTaO3 are observed in the temperature range from 14 to 1200 K, which includes the phase-tran sition temperature at about 890 K. In the ferroelectric phase, no evid ence of the softening of the A1 phonon is found in the spectra. Howeve r, the hyper-Raman spectra show a strong Debye-type relaxational mode in both phases. The hyper-Raman spectra clearly show that the phase tr ansition of LiTaO3 is of the order-disorder type. The anomalous line s hape of an A1 phonon band at 203 cm-1 is interpreted in terms of the c oupling between this phonon and the relaxational modes. The coupling m odel also explains the difference between the line shapes of the Raman and hyper-Raman spectra.