[Zurück]

P. Tomes, M. Ikeda, Gaku Eguchi, M. Baitinger, H. Nguyen, C. Allio, C. Krellner, Y. Grin, S. Paschen:
"Anisotropic heat and charge transport in the cubic clathrate Ba8Au6−xGe40+x";
Vortrag: Japan Physical Society 2016 fall meeting, Kanazawa, Japan; 13.09.2016 - 16.09.2016.

According to the phonon-glass electron-crystal concept [1], a good thermoelectric material exhibits low phonon thermal conductivity and high electrical conductivity. Type-I clathrates and filled skutterudites show remarkably low phonon thermal conductivity. Although its micro- scopic origin is still a matter of debate [2, 3, 4], there is a wide consensus that the cage-like crys- tal structure is of central importance to this phenomenon. The type-I clathrate Ba8Au6−xGe40+x possesses six tetrakaidecahedra and two dodecahedra cages per unit cell, and each cage encapsu- lates a weakly-bonded Ba atom. This material is particularly interesting for phonon transport studies because the rattling motion of Ba is associated with a very low characteristic energy.
In this presentation we report physical properties of single-crystalline Ba8Au5.3Ge40.7 includ- ing thermal conductivity, thermopower, specific heat, and electrical transport. Remarkably, the phonon thermal conductivity shows anisotropic behavior in spite of the cubic crystal symmetry. First principles calculations reveal that the anisotropic behavior can be attributed to unique properties of the electronic band structure. Phonon-hole coupling is found to be important espe- cially at low-temperatures, resulting in a smaller scattering probability along the [110] direction. Anisotropic electrical transport was also observed and was analyzed using a recently-proposed analysis method for a two-carrier model [5]. This analysis reveals a smaller mobility along [110]. We present details of the anisotropic behavior, and discuss relations between the phonon and electrical transport properties.