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

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.

10 pages, 10 figures

References in corpus (1)

• Low-lying optical phonon modes in the filled skutterudite CeRu4Sb12

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