Origin of the "Waterfall" Effect in Phonon Dispersion of Relaxor Perovskites

TL;DR

This study investigates the origin of the 'waterfall' effect in phonon dispersion of relaxor perovskites, revealing it depends on Brillouin zone choice and is explained by a coupled oscillator model, challenging previous size-based explanations.

Contribution

It demonstrates that the waterfall effect's position is zone-dependent and not directly related to polar nanoregion size, providing a new interpretation through a coupled oscillator model.

Findings

Waterfall position varies with Brillouin zone choice.

The effect is not correlated with polar nanoregion size.

A coupled oscillator model explains the phenomenon.

Abstract

Inelastic neutron scattering study of the perovskite relaxor ferroelectric PZN:8%PT elucidates the origin of the previously reported unusual kink on the low frequency transverse phonon dispersion curve (known as "waterfall" effect). We show that its position depends on the choice of the Brillouin zone and that the relation of its position to the size of the polar nanoregions is highly improbable. The observation is explained in the framework of a simple model of coupled damped harmonic oscillators representing the acoustic and optic phonon branches.