Dynamical effects of the nanometer-sized polarized domains in Pb(Zn1/3Nb2/3)O3 -: art. no. 224109

Results from an extensive single-crystal neutron-scattering study of the relaxer ferroelectric Pb(Zn1/3Nb2/3)O-3 (PZN) in the cubic phase reveal an anomalous ridge of inelastic scattering that extends vertically from the transverse acoustic (TA) branch near 4 meV to the transverse optic (TO) branch near 9 meV at q similar to0.2 Angstrom (-1). No zone center optic mode was found. Similar results were recently reported in a sample of PZN doped with 8% PbTiO3. We are able to describe the dynamics of this unusual feature using a simple coupled-mode model that assumes the optic mode linewidth Gamma (1)(q) increases sharply near q = 0.2 Angstrom (-1) as q -->0. The dramatic increase in Gamma (1)(q) is believed to occur when the wavelength of the optic mode becomes comparable to the size of small polarized nanoregions (PNR) that develop in the cubic phase of PZN, far above T-c=410 K. The consequence is that long-wavelength optic modes cannot propagate and become overdamped. Hence the q at which the anomalous scattering is peaked provides an important measure of the size of the PNR. Below T-c, the intensity of this scattering diminishes. At lowest temperatures (similar to 30 K) the feature is absent, and we observe the recovery of a zone center transverse optic mode near 10.5 meV.