Temperature Dependence of Raman and Rayleigh Scattering in LiNb O3 and LiTa O3

The Raman spectra of LiNbO₃ and LiTaO₃ have been investigated between 90 and 1100^∘{}K. The dependence of optic-mode frequencies and Raman-scattering efficiencies on temperature indicates that the ferroelectric transition (Tc≈900^∘K for LiTaO₃, {≈}1480^∘{}K for LiNbO₃) is second order and is associated with an optic phonon mode whose frequency becomes small as the Curie temperature is approached from below. Critical Rayleigh scattering is observed for LiTaO₃ and the shape of the anomaly is attributed to the combined effects of lattice polarization fluctuation and the discontinuity in electro-optic effect at the transition. The absolute scattering efficiency and linewidth data indicate that the stimulated Raman gain for several lattice modes is comparable with or greater than that for the 655-cm^-1 CS₂ line, and calculations show that cw Raman oscillation in these modes is feasible.