Soft phonon columns on the edge of the Brillouin zone in the relaxor PbMg1/3Nb2/3O3

TL;DR

This study uses neutron scattering to observe unusual phonon scattering columns in relaxor ferroelectric PbMg1/3Nb2/3O3, revealing complex lattice dynamics and a competition between different distortions near the zone edge.

Contribution

It provides the first detailed analysis of phonon columns in an insulating relaxor ferroelectric, proposing a model for atomic displacements and suggesting disorder influences these unique features.

Findings

Columns of phonon scattering are localized in momentum but extended in energy.

Phonon columns soften around 400 K, indicating a link to diffuse scattering onset.

Displacements involve Pb^{2+} and O^{2-} ions, not tilt modes.

Abstract

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.