We report neutron inelastic scattering on single crystal
0.68Pb(Mg1/3Nb2/3O3)-0.32PbTiO3 (PMN-0.32PT), a relaxor ferroelectric material
that lies within the compositional range of the morphotropic phase boundary
(MPB). Data were obtained between 100 K and 600 K under zero and non-zero
electric field applied along the cubic [001] direction. The lowest energy,
zone-center, transverse optic phonon is strongly damped and softens slowly at
high temperature; however the square of the soft mode energy begins to increase
linearly with temperature as in a conventional ferroelectric, which we term the
soft mode "recovery," upon cooling into the tetragonal phase at TC. Our data
show that the soft mode in PMN-0.32PT behaves almost identically to that in
pure PMN, exhibiting the same temperature dependence and recovery temperature
even though PMN exhibits no well-defined structural transition (no TC). The
temperature dependence of the soft mode in PMN-0.32PT is also similar to that
in PMN-0.60PT; however in PMN-0.60PT the recovery temperature equals TC. These
results suggest that the temperature dependence and the energy scale of the
soft mode dynamics in PMN-xPT are independent of concentration on the Ti-poor
side of the MPB, but scale with TC for Ti-rich compositions. Thus the MPB may
be defined in lattice dynamical terms as the concentration where TC first
matches the recovery temperature of the soft mode. High-resolution x-ray
studies show that the cubic-to-ferroelectric phase boundary shifts to higher
temperatures by an abnormal amount within the MPB region in the presence of an
electric field. This suggests that an unusual instability exists within the
apparently cubic phase at the MPB.Comment: 13 pages, 6 figure
