Hyper-Raman and Raman studies on the phase transition of ferroelectricLiTaO3

Hyper-Raman and Raman spectra of ferroelectric ${\mathrm{LiTaO}}_3$ are observed in the temperature range from 14 to 1200 K, which includes the phase-transition temperature at about 890 K. In the ferroelectric phase, no evidence of the softening of the ${\mathit{A}}_1$ phonon is found in the spectra. However, the hyper-Raman spectra show a strong Debye-type relaxational mode in both phases. The hyper-Raman spectra clearly show that the phase transition of ${\mathrm{LiTaO}}_3$ is of the order-disorder type. The anomalous line shape of an ${\mathit{A}}_1$ phonon band at 203 ${\mathrm{cm}}^{\mathrm{-}1}$ is interpreted in terms of the coupling between this phonon and the relaxational modes. The coupling model also explains the difference between the line shapes of the Raman and hyper-Raman spectra.