Possible mechanism for achieving glass-like thermal conductivities in crystals with off-center atoms

TL;DR

This paper investigates how off-center atoms in crystals influence thermal conductivity, revealing that off-center displacement enhances phonon scattering and can lead to glass-like thermal properties.

Contribution

It identifies off-center displacement as a key factor in phonon scattering, explaining differences in thermal conductivity among filled clathrates with similar Einstein modes.

Findings

Off-center atoms increase phonon scattering in Eu and Sr systems.

Presence of off-center sites introduces symmetry-breaking defects.

Coupling between rattler motion and phonons is enhanced by off-center displacement.

Abstract

In the filled Ga/Ge clathrate, Eu and Sr are off-center in site 2 but Ba is on-center. All three filler atoms (Ba,Eu,Sr) have low temperature Einstein modes; yet only for the Eu and Sr systems is there a large dip in the thermal conductivity, attributed to the Einstein modes. No dip is observed for Ba. Here we argue that it is the off-center displacement that is crucial for understanding this unexplained difference in behavior. It enhances the coupling between the "rattler" motion and the lattice phonons for the Eu and Sr systems, and turns on/off another scattering mechanism (for 1K < T < 20K) produced by the presence/absence of off-center sites. The random occupation of different off-center sites produces a high density of symmetry-breaking defects which scatters phonons. It may also be important for improving our understanding of other glassy systems.