NMR relaxation and rattling phonons in type-I Ba8Ga16Sn30 clathrate

TL;DR

This study uses NMR to investigate how guest atom rattling within type-I Ba8Ga16Sn30 clathrates influences thermal properties, revealing a relaxation peak linked to anharmonic rattling phonons.

Contribution

It provides a detailed NMR analysis of guest atom rattling dynamics and models these using a two-phonon Raman process with anharmonic oscillators, advancing understanding of thermal behavior in clathrates.

Findings

Identification of a low-temperature relaxation peak due to rattling dynamics.

Validation of a two-phonon Raman model for guest atom vibrations.

Parameters indicating large anharmonicity in localized oscillators.

Abstract

Atomic motion of guest atoms inside semiconducting clathrate cages is considered as an important source for the glasslike thermal behavior.69Ga and 71Ga Nuclear Magnetic Resonance (NMR) studies on type-I Ba8Ga16Sn30 show a clear low temperature relaxation peak attributed to the influence of Ba rattling dynamics on the framework-atom resonance, with a quadrupolar relaxation mechanism as the leading contribution. The data are analyzed using a two-phonon Raman process, according to a recent theory involving localized anharmonic oscillators. Excellent agreement is obtained using this model, with the parameters corresponding to a uniform array of localized oscillators with very large anharmonicity.