| Résumé | We report lattice-dynamical measurements, made using neutron inelastic-scattering methods, of the relaxor perovskite PbMg1/3Nb2/3O3PMN 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:QR=1 2 , 1 2 , 1 2and QM=12,12,0.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 le 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. |
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