  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.

Materials Science