Ferroelectric dynamics in the perovskite relaxor PMN

We review results obtained from recent neutron scattering studies of the lead-oxide class of perovskite relaxors PMN and PZN. A ferroelectric soft mode has been identified in PMN at 1100 K that becomes overdamped near 620 K. This is the same temperature at which polar nanoregions (PNR) begin to form, denoted by Td, and suggests that a direct connection exists between the soft mode and the PNR. The appearance of diffuse scattering intensity at Td reported by Naberezhnov et al. lends further support to this picture. At lower temperature the soft mode in PMN reappears close to Tc = 213 K (defined only for E > Ec). These results are provocative because the dynamics below Tc are characteristic of an ordered ferroelectric state, yet they occur in a system that remains cubic on average at all temperatures. We discuss a coupled-mode model that successfully describes these data as well as those from earlier lattice dynamical studies of other perovskites such as BaTiO3