Dynamic origin of the morphotropic phase boundary:: Soft modes and phase instability in 0.68Pb(Mg1/3Nb2/3O3)-0.32PbTiO3

We report neutron inelastic scattering and high-resolution x-ray diffraction measurements on single crystal 0.68Pb(Mg1/3Nb2/3O3)-0.32PbTiO(3)) (PMN-0.32PT), a relaxor ferroelectric material that lies within the compositional range of the morphotropic phase boundary (MPB) where the piezoelectric properties of PMN-xPT compounds are close to maximal. Data were obtained between 100 and 600 K under zero and nonzero 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 T-C. 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 T-C). 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 T-C. 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 T-C for Ti-rich compositions. Thus the MPB may be defined in lattice dynamical terms as the concentration where T-C 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.