We report neutron inelastic scattering experiments on single-crystal PbMg1/3Nb2/3O3 doped with 32% PbTiO3, a relaxor ferroelectric that lies close to the morphotropic phase boundary. When cooled under an electric field E∥ [001] into tetragonal and monoclinic phases, the scattering cross section from transverse acoustic (TA) phonons polarized parallel to E weakens and shifts to higher energy relative to that under zero-field-cooled conditions. Likewise, the scattering cross section from transverse optic (TO) phonons polarized parallel to E weakens for energy transfers 4≤ω≤9 meV. However, TA and TO phonons polarized perpendicular to E show no change. This anisotropic field response is similar to that of the diffuse scattering cross section, which, as previously reported, is suppressed when polarized parallel to E but not when polarized perpendicular to E. Our findings suggest that the lattice dynamics and dynamic short-range polar correlations that give rise to the diffuse scattering are coupled.

We present neutron inelastic scattering measurements of the low-energy phonons in single crystal BiFeO3. The dispersions of the three acoustic phonon modes (LA along [100], TA1 along [010], and TA2 along [11¯0]) and two low-energy optic phonon modes (LO and TO1) have been mapped out between 300 and 700K. Elastic constants are extracted from the phonon measurements. The energy linewidths of both TA phonons at the zone boundary clearly broaden when the system is warmed toward the magnetic ordering temperature TN=640K. This suggests that the magnetic order and low-energy lattice dynamics in this multiferroic material are coupled.