College Park, Maryland June 6 - 10 , 2004
M3-C1 (1:30 PM): Dynamic Properties of the Relaxors Pb(Mg1/3Nb2/3)O3 and Pb(Zn1/3Nb2/3)O3 (Invited)
C. Stock , R. J. Birgeneau, S. Wakimoto (Physics Department , University of Toronto, Toronto, Ontario, Canada), G. Shirane (Physics Department, Brookhaven National Laboratory)
The relaxors Pb(Mg1/3Nb2/3)O3 (PMN) and Pb(Zn1/3Nb2/3)O3 (PZN) both exhibit a very similar dielectric response around the critical temperature Tc. Recent neutron inelastic scattering results have found a recovery of the transverse optic mode below Tc in both materials, indicating that a ferroelectric distortion exists in the unpoled state. It was widely believed that these two materials were very different based on x-ray diffraction results, which pointed to PZN as being rhombohedral at low temperatures while PMN remained cubic. Recent high-energy x-ray experiments have suggested, however, that the bulk low-temperature unit cells in both PMN and PZN are nearly cubic, and have found that the near-surface unit cell in PZN is rhombohedral. Because of this outcome, a detailed comparison of the dynamic and static properties of PMN and PZN is required in order reveal their similarities and differences.
We present a review of neutron scattering results showing that the static and dynamic properties of PMN and PZN are very similar. We define the similarity through a comparison of the low-frequency phonons and the diffuse scattering observed in both systems. Based on these similarities we propose that these materials belong to the same universality class. We argue that the three-dimensional Heisenberg model, with cubic anisotropy, in a random field may explain many of the unusual properties observed in PMN and PZN and compare the static and dynamic properties to those of Mn0.5Zn0.5F2. We also discuss results suggesting that the diffuse scattering and the anomalous damping of the optic mode can be consistently understood, and are not simply artifacts of coupling between the acoustic and optic modes.
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