Significance of Off-Center Rattling for Emerging Low-lying THz Modes in type-I Clathrates

TL;DR

This paper investigates how off-center guest ions in type-I clathrates significantly alter low-frequency THz dynamics, leading to glass-like thermal conductivities due to flattened phonon dispersion.

Contribution

It introduces a theoretical model that explains the distinct THz dynamics and thermal properties of off-center versus on-center clathrates, highlighting the role of rattling guest ions.

Findings

Off-center guest ions cause a drastic change in THz-frequency dynamics.

Glass-like thermal conductivities are linked to flattened phonon dispersion.

The mechanism applies to glasses and relaxers as well.

Abstract

We show that the distinct differences of low-lying THz-frequency dynamics between type-I clathrates with on-center and off-center guest ions naturally follow from a theoretical model taking into account essential features of the dynamics of rattling guest ions. Our model analysis demonstrates the drastic change from the conventional dynamics shown by on-center systems to the peculiar dynamics of off-center systems in a unified manner. We claim that glass-like plateau thermal conductivities observed for off-center systems stem from the flattening of acoustic phonon dispersion in the regime |k|<|G|/4. The mechanism is applicable to other systems such as glasses or relaxers.