Inelastic neutron scattering study of the relaxor ferroelectric PbMg

Results of inelastic neutron scattering experiments between 300 K and 900 K on the relaxor ferroelectric PbMg1/3Nb2/3O3 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 ${ T}_{\rm d}\approx 650$ 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 q4. Different physical models of the crossover are discussed