Frequency Dependence of the Damping Function of the SoftE-Symmetry Phonon in PbTiO3

We have measured, by means of Raman scattering from phonon polaritons, the frequency dependence of the damping function $\gamma (\omega , T_{\mathrm{rm}})$ of the soft $E$-symmetry phonon in PbTi${\mathrm{O}}_3$ at room temperature. Our result shows that the divergence of $\gamma (\omega , T)$ for $T\to T_c$ arises mainly from the frequency dependence of $\gamma (\omega , T)$ and that $\gamma (\omega , T)$ for a fixed frequency $\omega$ is simply proportional to the temperature. The apparent divergence of $\gamma (\omega , T)$ for $T\to T_c$ observed by Burns and Scott results from the temperature dependence of the soft-mode frequency.