Structure and Thermoelectric Properties of Type-I Clathrates Ba8(AlxGa1−x)16Ge30

Licentiate thesis, 2020

Thermoelectric materials enable the direct conversion between a thermal gradient and an electrical potential gradient, and can thus be exploited for electricity generation. One of the promising thermoelectric materials is type-I clathrates. They are regarded as realizations of the phonon-glass electron-crystal concept, combining relatively large electrical conductivity with very low thermal conductivity. However, a moderate power factor renders room for further improvement, and the role of the complex atomic structure has not been fully understood. This thesis studies the thermoelectric properties of type-I clathrate Ba8(AlxGa1−x)16Ge30 and investigates the impact of structure on the electron and phonon transport.
The power factor of clathrates is improved by modulation doping by introducing analogous compound Ba8Al16Ge30 to the matrix phase Ba8Ga16Ge30. A heterogeneous microstructure is created in the materials, which are composed of a Ga-rich clathrate matrix phase and Al particles. The charge carriers transfer from the latter to the former phase without lowering the charge carrier mobility, as compared to homogeneously doped materials. As a result of modulation doping, the carrier concentration and carrier mobility increase simultaneously. Especially, the carrier mobility is enhanced to a value that exceeds that reported for a single crystal with the same composition. Consequently, the highest figure of merit (zT) is achieved for Ba8(Al0.25Ga0.75)16Ge30, with the maximum value reaching 0.93 at 800 °C.
The atomic structure of quaternary Ba8(AlxGa1−x)16Ge30 is studied by the combination of X-ray and neutron diffraction. The obtained chemical ordering is validated and consistent with theoretical calculations. It is found that depending on the synthesis method, the trivalent elements Al and Ga show different occupation in the host framework. In turn, the atomic displacement parameter of the guest atoms in the larger tetrakaidecahedral cages is influenced by chemical ordering: the guest atoms are either localized at the center of the cage or moving towards the boundary of the cage periphery.