Optical conductivity of rattling phonons in type-I clathrate Ba$_8$Ga$_{16}$Ge$_{30}$

TL;DR

This study investigates the infrared-active rattling phonons in Ba8Ga16Ge30 using terahertz spectroscopy, revealing temperature-dependent spectral changes consistent with anharmonic phonon models.

Contribution

It provides the first detailed experimental analysis of rattling phonons in this clathrate, confirming anharmonic potential effects on phonon spectra.

Findings

Detection of low-lying rattling phonons at 1.15 and 1.80 THz

Temperature-dependent spectral sharpening and softening consistent with anharmonic models

Spectral changes explained by Bose-factor without interaction effects

Abstract

A series of infrared-active optical phonons have been detected in type-I clathrate Ba$_8$Ga$_{16}$Ge$_{30}$ by terahertz time-domain spectroscopy. The conductivity spectra with the lowest-lying peaks at 1.15 and 1.80 THz are identified with so-called rattling phonons, i.e., optical modes of the guest ion Ba$^{2+}(2)$ with $T_{1u}$ symmetry in the oversized tetrakaidecahedral cage. The temperature dependence of the spectra from these modes are totally consistent with calculations based on a one-dimensional anharmonic potential model that, with decreasing temperature, the shape becomes asymmetrically sharp associated with a softening for the weight to shift to lower frequency. These temperature dependences are determined, without any interaction effects, by the Bose-factor for optical excitations of anharmonic phonons with the nonequally spaced energy levels.