Soft phonon columns on the edge of the Brillouin zone in the relaxor PbMg_1/3Nb_2/3O₃

We report lattice-dynamical measurements, made using neutron inelastic-scattering methods, of the relaxor perovskite PbMg_1/3Nb_2/3O₃ (PMN) at momentum transfers near the edge of the Brillouin zone. Unusual “columns” of phonon scattering that are localized in momentum, but extended in energy, are seen at both high-symmetry points along the zone edge: Q⃗_R={1/2,1/2,1/2} and Q⃗_M={1/2,1/2,0}. These columns soften at ∼400 K which is similar to the onset temperature of the zone-center diffuse scattering, indicating a competition between ferroelectric and antiferroelectric distortions. We propose a model for the atomic displacements associated with these phonon modes that is based on a combination of structure factors and group theoretical analysis. This analysis suggests that the scattering is not from tilt modes (rotational modes of oxygen octahedra), but from zone-boundary optic modes that are associated with the displacement of Pb²⁺ and O²⁻ ions. Whereas similar columns of scattering have been reported in metallic and (less commonly) molecular systems, they are unusual in insulating materials, particularly in ferroelectrics; therefore, the physical origin of this inelastic feature in PMN is unknown. We speculate that the underlying disorder contributes to this unique anomaly.