Soft Phonon Anomalies in the Relaxor Ferroelectric   Pb(Zn_1/3Nb_2/3)_0.92Ti_0.08O_3
  Neutron inelastic scattering measurements of the polar TO phonon mode dispersion in the cubic relaxor Pb(Zn_1/3Nb_2/3)_0.92Ti_0.08O_3 at 500K reveal anomalous behavior in which the optic branch appears to drop precipitously into the acoustic branch at a finite value of the momentum transfer q=0.2 inverse Angstroms, measured from the zone center. We speculate this behavior is the result of nanometer-sized polar regions in the crystal. 
P. M. Gehring, S. E. Park, G. Shirane