Analysis of quasielastic light scattering in ${\mathrm{LiTaO}}_3$ near $T_C$

We have reexamined the Raman spectra of lithium tantalate from 294 to 1059 K with special emphasis upon the quasielastic scattering from 0–200 ${\mathrm{cm}}^{\mathrm{-}1}$. We show that the complete $A_1$-symmetry spectrum at each temperature, including both the lowest-energy optical phonon and the quasielastic scattering, can be fitted to the response function for a system with a relaxing self-energy ${\chi }^{\mathrm{-}1}$(ω,T)=${\omega }_0^2$(T)-ω ${}_{}{}^2{}_{}{}^{}$-iωγ(T)-${\delta }^2$(T)/[1-iωτ(T)] and that the fitting parameters vary monotonically with temperature. In agreement with recent analysis of ${\mathrm{LiNbO}}_3$ [Okamoto, Wang, and Scott, Phys. Rev. B 32, 6787 (1985)] we find that the linewidth γ for the lowest energy $A_1$ phonon increases rapidly with temperature, from 27 ${\mathrm{cm}}^{\mathrm{-}1}$ at 294 K to 382 ${\mathrm{cm}}^{\mathrm{-}1}$ very near $T_C$; τ exhibits a critical slowing down from 0.50 ps far from $T_C$ to 2.6 ps at $T_C$ and continues to increase above $T_C$ (5.3 ps at 1059 K); the lowest-energy optical phonon ‘‘softens’’ only slightly, from ${\omega }_0$ of 202 ${\mathrm{cm}}^{\mathrm{-}1}$ at 294 K to 178 ${\mathrm{cm}}^{\mathrm{-}1}$ at $T_C$=881 K (determined from the dielectric constant), and that the coupling constant ${\delta }^2$(T) increases supralinearly with temperature, as in ${\mathrm{LiNbO}}_3$.