  We report lattice dynamical measurements, made using neutron inelastic
scattering methods, of the relaxor perovskite PbMg1/3Nb2/3O3 (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: \vec{Q}_R={1/2, 1/2, 1/2} and
\vec{Q}_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 Pb^{2+} and 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.
