Preliminary results of lattice-dynamical shell-model calculations for the relaxor ferroelectric PbMg1/3Nb2/3O3 (PMN) in the virtual-ion approximation are presented. The model parameters are fitted to Raman scattering and low-frequency inelastic neutron scattering data, and higher-frequency dispersion curves are predicted.

A detailed lattice-dynamic investigation on the relaxor ferroelectric PMN was started to search for "additional" modes (dispersion curves). But the results are complex with 4 branches observed below 6.5 THz, a broad unresolved continuum between 6.5 and 12 THz, a well defined excitation at 13.4 THz with longitudinal and transverse character and further weak signals above 16 THz. Possible effects caused by disorder are discussed.

Results of inelastic neutron scattering experiments between 300 K and 900 K on the relaxor ferroelectric PbMg 1/3 Nb 2/3 O 3 are presented. Within a mode-coupling analysis the data are consistent with the observation of a strongly damped quasi-optic excitation. It is demonstrated that below T d ≈ 650 K a dynamical crossover takes place manifesting itself by a narrow central peak. This crossover is accompanied by the appearance of strong damping of the transverse acoustic phonons, with the damping constant proportional to q 4 . Different physical models of the crossover are discussed.