Origin of the “Waterfall” Effect in Phonon Dispersion of Relaxor Perovskites

We have undertaken an inelastic neutron scattering study of the perovskite relaxor ferroelectric [Formula presented] with 8% [Formula presented] (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 ([Formula presented]) depends on the choice of the Brillouin zone and that the relation of [Formula presented] 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.