Glasslike vs. crystalline thermal conductivity in carrier-tuned Ba8Ga16X30 clathrates (X = Ge, Sn)

TL;DR

This study investigates how charge carriers influence the thermal conductivity in Ba8Ga16X30 clathrates, revealing that carrier type affects whether the material exhibits crystalline or glasslike thermal behavior, linked to vibrational mode coupling.

Contribution

It demonstrates that the transition from crystalline to glasslike thermal conductivity depends on carrier type and vibrational coupling, providing a unified explanation for thermal transport in these materials.

Findings

Electrons lead to crystalline peak in k(T)

Holes cause glasslike plateau in k(T)

Coupling strength between vibrational modes influences behavior

Abstract

The present controversy over the origin of glasslike thermal conductivity observed in certain crystalline materials is addressed by studies on single-crystal x-ray diffraction, thermal conductivity k(T) and specific heat Cp(T) of carrier-tuned Ba8Ga16X30 (X = Ge, Sn) clathrates. These crystals show radically different low-temperature k(T) behaviors depending on whether their charge carriers are electrons or holes, displaying the usual crystalline peak in the former case and an anomalous glasslike plateau in the latter. In contrast, Cp(T) above 4 K and the general structural properties are essentially insensitive to carrier tuning. We analyze these combined results within the framework of a Tunneling/Resonant/Rayleigh scatterings model, and conclude that the evolution from crystalline to glasslike k(T) is accompanied by an increase both in the effective density of tunnelling states and in the resonant scattering level, while neither one of these contributions can solely account for the observed changes in the full temperature range. This suggests that the most relevant factor which determines crystalline or glasslike behavior is the coupling strength between the guest vibrational modes and the frameworks with different charge carriers.