Cage-size control of guest vibration and thermal conductivity in Sr8Ga16Si30-xGex

TL;DR

This study investigates how varying the composition of Sr8Ga16Si30-xGex affects guest vibrations and thermal conductivity, revealing a transition from crystalline to glass-like thermal behavior linked to guest ion dynamics.

Contribution

It provides a systematic analysis of guest vibrational modes and thermal transport in Sr8Ga16Si30-xGex, highlighting the impact of guest off-center vibrations on thermal conductivity.

Findings

Thermal conductivity decreases with increasing Ge content.

Localized vibrational energies of guest ions decrease as x increases.

Transition from crystalline to glass-like thermal behavior observed for x > 20.

Abstract

We present a systematic study of thermal conductivity, specific heat, electrical resistivity, thermopower and x-ray diffraction measurements performed on single-crystalline samples of the pseudoquaternary type-I clathrate system Sr8Ga16Si30-xGex, in the full range of 0 < x < 30. All the samples show metallic behavior with n-type majority carriers. However, the thermal conductivity and specific heat strongly depend on x. Upon increasing x from 0 to 30, the lattice parameter increases by 3%, from 10.446 to 10.726 A, and the localized vibrational energies of the Sr guest ions in the tetrakaidekahedron (dodecahedron) cages decrease from 59 (120) K to 35 (90) K. Furthermore, the lattice thermal conductivity at low temperatures is largely suppressed. In fact, a crystalline peak found at 15 K for x = 0 gradually decreases and disappears for x > 20, evolving into the anomalous glass-like behavior observed for x = 30. It is found that the increase of the free space for the Sr guest motion directly correlates with a continuous transition from on-center harmonic vibration to off-center anharmonic vibration, with consequent increase in the coupling strength between the guest's low-energy modes and the cage's acoustic phonon modes.