Abstract
The intrinsic linewidth ${\ensuremath{\Gamma}}_{\mathrm{TA}}$ of the transverse acoustic phonon observed in the relaxor ferroelectric compound $\mathrm{Pb}({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}{)}_{0.8}{\mathrm{Ti}}_{0.2}{\mathrm{O}}_{3}$ (PMN-20%PT) begins to broaden with decreasing temperature around 650 K, nearly 300 K above the ferroelectric transition temperature ${T}_{c} (\ensuremath{\sim}360 \mathrm{K}).$ We speculate that this anomalous behavior is directly related to the condensation of polarized, nanometer-sized, regions (PNR) at the Burns temperature ${T}_{d}.$ We also observe the ``waterfall'' anomaly previously seen in pure PMN, in which the transverse optic branch appears to drop precipitously into the TA branch at a finite momentum transfer ${q}_{\mathrm{wf}}\ensuremath{\sim}0.15 {\mathrm{\AA{}}}^{\ensuremath{-}1}.$ The waterfall feature is seen even at temperatures above ${T}_{d}.$ This latter result suggests that the PNR exist as dynamic entities above ${T}_{d}.$
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