Origin of the "Waterfall" effect in phonon dispersion of relaxor perovskites

We have undertaken an inelastic neutron scattering study of the perovskite relaxor ferroelectric P b ( Z n 1 / 3 N b 2 / 3 ) O 3 with 8% P b T i 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 w f ) depends on the choice of the Brillouin zone and that the relation of q w 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.