Assignment and temperature dependence of the Raman modes ofLiTaO3studied over the ferroelectric and paraelectric phases

The Raman spectra of single crystals of ${\mathrm{LiTaO}}_3$ have been studied over two phases in the temperature range 290–1200 K. Assignments of normal Raman modes in both the ferroelectric and paraelectric phases are given using data from several scattering configurations and taking into account the directional properties (in the ferroelectric phase) and temperature dependence of observed phonons. The number of normal Raman modes observed in each phase (13 optical branches in the ferroelectric phase and 5 in the paraelectric phase) indicates a transition from a $C_{3v}$ to a $D_{3d}$ symmetry, a result that is in agreement with diffraction data. Most modes in the ferroelectric phase soften with increasing temperature, the only exceptions being three E symmetry modes which are temperature independent. In the paraelectric phase three modes soften with increasing temperature, while the other two appear to be T independent. The manner in which the Raman spectra vary with temperature supports the theory of the order-disorder mechanism for the transition and also suggests that an ‘‘intermediate phase’’ exists (mixing of the $C_{3v}$ and $D_{3d}$ symmetries) within a temperature range of about 100 K below the critical point.