Soft phonon columns on the edge of the Brillouin zone in the relaxor PbMg 1 / 3 Nb 2 / 3 O 3 </mml:…
2009; American Physical Society; Volume: 79; Issue: 22 Linguagem: Inglês
10.1103/physrevb.79.224301
ISSN1550-235X
AutoresI. P. Swainson, Christopher Stock, P. M. Gehring, Guangyong Xu, K. Hirota, Y. Qiu, H. S. Luo, Xiangyong Zhao, J.-F. Li, D. Viehland,
Tópico(s)Multiferroics and related materials
ResumoWe 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: ${\stackrel{P\vec}{Q}}_{R}={\frac{1}{2},\frac{1}{2},\frac{1}{2}}$ and ${\stackrel{P\vec}{Q}}_{M}={\frac{1}{2},\frac{1}{2},0}$. These columns soften at $\ensuremath{\sim}400\text{ }\text{K}$ 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 ${\text{O}}^{2\ensuremath{-}}$ 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.
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