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Anisotropic heat and charge transport in the cubic clathrate Ba8Au6-xGe40+x.

A good thermoelectric material exhibits, in addition to large thermopower, low phonon thermal conductivity high electrical conductivity. Type-I clathrates, which show promise for thermoelectric application, exhibit remarkably low phonon thermal conductivities, and there is wide consensus that the cage-like crystal 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 encapsulates 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 characteristic energy [Pierre-Franc¸ois Lory et al., Nat. Commun. 8, 491 (2017)]. In this presentation we report electrical and thermal transport properties of single-crystalline Ba7:81Au5:33Ge40:67. Remarkably, both the electrical resistivity and the phonon thermal conductivity show pronounced anisotropy. We observed a reduced electrical conductivity but an enhanced phonon thermal conductivity along the [110] direction. Electronic band structure calculations reveal pronounced anisotropy for the same direction.We suggest that strong electron-phonon coupling is at the origin of the observed phenomena.

Anisotropic heat and charge transport, cubic clathrate Ba8Au6-xGe40+x

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