Host structure engineering in thermoelectric clathrates

Three Ba8Al16Ge30 clathrate type I samples have been prepared using three different synthesis methods; flux growth, Czochralski growth, and stoichiometric mixing. The samples were characterized by X-ray powder diffraction, multi-temperature single-crystal X-ray diffraction, high-resolution low-temperature (20 K) single-crystal neutron diffraction, and measurement of transport properties. The samples show a remarkable variation in the aluminum/germanium occupancies on the host-structure sites and minor variation in the total aluminum content. The observed occupancy variation forms the basis for the formulation of a set of simple rules for the maximum site occupancy factors of trivalent elements in the host structure. The rules provide an explanation for why the overwhelming majority of clathrate samples containing trivalent elements are n-type rather than p-type. The nuclear density of the barium guest atom in the large cage is calculated from the neutron diffraction data, and it is found to be strongly dependent on the host structure and the exact aluminum siting. Thus, the sample with low aluminum content has a prolate-shaped barium nuclear density, whereas a higher aluminum content leads to the well-known torus shape observed in other clathrates. This demonstrates that the host-guest interactions are not merely ionic and that they significantly influence the guest-atom structure and dynamics. Comparison of derived Einstein temperatures and bond distances for the three samples reveals that the compound with the highest aluminum content (flux growth) has the strongest host-guest interaction, even though it also has the largest unit cell. Again, the specific properties of the guest atoms are not only determined by the clathrate cage size but also by the subtle chemical interactions between the host structure and the guest. The thermal conductivity is about 3 times smaller for the stoichiometric sample than for the Czochralskipulled sample. Thus, control of the host-structure chemistry is not only a key to manipulating the electrical properties of clathrates but also the thermal conductivity.