Phononic filter effect of rattling phonons in the thermoelectric clathrate Ba$_8$Ge$_{40+x}$Ni$_{6-x}$

TL;DR

This study combines neutron scattering and theoretical calculations to reveal how rattling phonons in Ba-Ge-Ni clathrates act as a low pass filter, reducing thermal conductivity and enhancing thermoelectric performance.

Contribution

It demonstrates the hybridization of guest and host phonons and interprets these as anti-crossings that function as a phononic low pass filter, a novel insight into thermal transport.

Findings

Hybridization between guest and host phonons occurs over a wide Brillouin zone.

Phononic anti-crossings act as a low pass filter for acoustic phonons.

The low pass filter effect reduces thermal conductivity in Ba-Ge-Ni clathrates.

Abstract

One of the key requirements for good thermoelectric materials is a low lattice thermal conductivity. Here we present a combined neutron scattering and theoretical investigation of the lattice dynamics in the type I clathrate system Ba-Ge-Ni, which fulfills this requirement. We observe a strong hybridization between phonons of the Ba guest atoms and acoustic phonons of the Ge-Ni host structure over a wide region of the Brillouin zone which is in contrast with the frequently adopted picture of isolated Ba atoms in Ge-Ni host cages. It occurs without a strong decrease of the acoustic phonon lifetime which contradicts the usual assumption of strong anharmonic phonon--phonon scattering processes. Within the framework of ab-intio density functional theory calculations we interpret these hybridizations as a series of an ti-crossings which act as a low pass filter, preventing the propagation of acoustic phonons. To highlight the effect of such a phononic low pass filter on the thermal transport, we compute the contribution of acoustic phonons to the thermal conductivity of Ba$_8$Ge$_{40}$Ni$_{6}$ and compare it to those of pure Ge and a Ge$_{46}$ empty-cage model system.