We have undertaken an inelastic neutron scattering study of the perovskite relaxor ferroelectric $\mathrm{P}\mathrm{b}({\mathrm{Z}\mathrm{n}}_{1/3}{\mathrm{N}\mathrm{b}}_{2/3}){\mathrm{O}}_3$ with 8% ${\mathrm{P}\mathrm{b}\mathrm{T}\mathrm{i}\mathrm{O}}_3$ (PZN-8%PT) in order to elucidate the origin of the previously reported unusual kink on the low frequency transverse phonon dispersion curve (known as the “waterfall effect”). We show that its position ($q_{\mathrm{w}\mathrm{f}}$) depends on the choice of the Brillouin zone and that the relation of $q_{\mathrm{w}\mathrm{f}}$ to the size of the polar nanoregions is highly improbable. The waterfall phenomenon is explained in the framework of a simple model of coupled damped harmonic oscillators representing the acoustic and optic phonon branches.

• Received 27 June 2003

DOI:https://doi.org/10.1103/PhysRevLett.91.107602