Universal van der Waals-type interactions in rattler containing cage materials

TL;DR

This paper reveals that van der Waals-type interactions universally explain the low-energy rattling modes in cage materials, impacting their thermal and electrical properties, especially in type-I clathrates, skutterudites, and pyrochlores.

Contribution

It demonstrates that van der Waals-type interactions are the primary origin of low-energy rattling modes in cage materials, providing a universal explanation across different structures.

Findings

Van der Waals interactions dominate between fillers and cages.

Mass, space, and chemical environment influence these interactions.

Universal rule applies to various cage materials.

Abstract

Rattling motion of fillers in cage materials has been of great interest for their import roles in superconductivity and thermoelectric applications. The standing waves of the rattling oscillations are normally lower in energy than the propagating waves of the acoustic phonons, thus exert large influences on the configuration of phonon dispersions as well as the associated thermal and electrical properties. Although it has been extensively studied, the origin of the low energy soft modes is still not clear. In the present paper, we show that van der Waals-type interactions are predominant between fillers and their surrounding cage frameworks, which explains the origin of the low energy modes in cage materials as a universal rule. Mass, free space and chemical environment of guest atoms are shown to be the most important factors to determine the three dimensional van der Waals-type interactions. The present work is mainly focused on type-I clathrates, skutterudites and pyrochlores.