  We present a quantitative analysis of the phonon lineshapes obtained by
neutron inelastic scattering methods in the relaxor ferroelectric
Pb(Mg1/3Nb2/3)O3 (PMN). Differences in the shapes and apparent positions of the
transverse acoustic (TA) and transverse optic (TO) phonon peaks measured in the
(300) and (200) Brillouin zones at 690 K are well described by a simple model
that couples the TA and soft TO modes in which the primary parameter is the
wave vector and temperature-dependent TO linewidth Gamma(q,T). This
mode-coupling picture provides a natural explanation for the uniform
displacements of the polar nanoregions, discovered by Hirota et al., as the PNR
result from the condensation of a soft TO mode that also contains a large
acoustic component.
