Raman Study of Cluster Dynamics in Disordered Ferroelectrics

We demonstrate for the first time that information on the dynamics of polar clusters in disordered ferroelectrics can be obtained from an analysis of the first order Raman scattering by hard phonon modes. The model developed allows one to separate the effect of the critical slowing down due to the divergence of the static dielectric susceptibility from that of the slowing down of the dynamics of local clusters. Raman results obtained on a ${\mathrm{K}}_{1-x}{\mathrm{TaNb}}_x{\mathrm{O}}_3$ crystal with $x=15.7\mathrm{}\%$ have revealed such a dichotomy of the cluster dynamics. The experimental results are consistent with the picture that, in disordered ferroelectrics, the slowing down of local clusters presents similarities to that in glasses and spin glasses.