We report lattice-dynamical measurements, made using neutron inelastic-scattering methods, of the relaxor perovskite ${\text{PbMg}}_{1/3}{\text{Nb}}_{2/3}{\text{O}}_3$ (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: ${\vec{Q}}_R=\left\{\frac{1}{2},\frac{1}{2},\frac{1}{2}\right\}$ and ${\vec{Q}}_M=\left\{\frac{1}{2},\frac{1}{2},0\right\}$. These columns soften at $\sim 400\text{K}$ 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 ${\text{Pb}}^{2+}$ and ${\text{O}}^{2-}$ 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.

• Received 31 October 2008

DOI:https://doi.org/10.1103/PhysRevB.79.224301