Anomalous transverse acoustic phonon broadening in the relaxor ferroelectric Pb(Mg1/3Nb2/3)(0.8)Ti0.2O3

The intrinsic linewidth Gamma(TA) of the transverse acoustic phonon observed in the relaxor ferroelectric compound Pb(Mg1/3Nb2/3)(0.8)Ti0.2O3 (PMN-20%PT) begins to broaden with decreasing temperature around 650 K, nearly 300 K above the ferroelectric transition temperature T-c (similar to360 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(wf)similar to0.15 Angstrom(-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.