Soft Phonon Columns on the Edge of the Brillouin Zone in the Relaxor PbMg1/3Nb2/3O3

Journal Article · Mon Jun 01 00:00:00 EDT 2009 · Physical Review. B, Condensed Matter and Materials Physics

DOI:https://doi.org/10.1103/PhysRevB.79.224301· OSTI ID:1040142

Swainson, I P; Xu, G; Stock, C; Gehring, P M; Hirota, K; Qiu, Y; Luo, H; Zhao, X; Li, J -F; Viehland, D

We report lattice-dynamical measurements, made using neutron inelastic-scattering methods, of the relaxor perovskite PbMg{sub 1/3}Nb{sub 2/3}O{sub 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: Q{sub R}={l_brace}1/2,1/2,1/2{r_brace} and Q{sub M}={l_brace}1/2,1/2,0{r_brace}. These columns soften at {approx}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{sup 2+} and O{sup 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.

• Research Organization: Brookhaven National Lab. (BNL), Upton, NY (United States)
• Sponsoring Organization: USDOE SC OFFICE OF SCIENCE (SC)
• DOE Contract Number: DE-AC02-98CH10886
• OSTI ID: 1040142
• Report Number(s): BNL-90405-2009-JA; R&D Project: PO-010; KC0202010; TRN: US1202322
• Journal Information: Physical Review. B, Condensed Matter and Materials Physics, Vol. 79, Issue 22; ISSN 1098-0121
• Country of Publication: United States
• Language: English

Related Subjects

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS
75 CONDENSED MATTER PHYSICS
SUPERCONDUCTIVITY AND SUPERFLUIDITY
ATOMIC DISPLACEMENTS
BRILLOUIN ZONES
DIFFUSE SCATTERING
INELASTIC SCATTERING
MOMENTUM TRANSFER
NEUTRONS
OPTICS
OXYGEN
PEROVSKITE
PHONONS
SCATTERING
STRUCTURE FACTORS
crystal structure
lead compounds
neutron diffraction
phonons
relaxor ferroelectrics
soft modes