| Abstract
| - Subtle changes in the host-structure stoichiometry of thermoelectric Ba8Al16-xGe30+x clathrates have profound consequences on the barium guest-atom structure as demonstrated in the nuclear density obtained from single crystal neutron diffraction. Control of the host-structure chemistry provides a route to manipulate both electrical and thermal properties of 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-raypowder 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 aremarkable variation in the aluminum/germanium occupancies on the host-structure sites and minorvariation in the total aluminum content. The observed occupancy variation forms the basis for theformulation of a set of simple rules for the maximum site occupancy factors of trivalent elements in thehost structure. The rules provide an explanation for why the overwhelming majority of clathrate samplescontaining trivalent elements are n-type rather than p-type. The nuclear density of the barium guest atomin the large cage is calculated from the neutron diffraction data, and it is found to be strongly dependenton the host structure and the exact aluminum siting. Thus, the sample with low aluminum content has aprolate-shaped barium nuclear density, whereas a higher aluminum content leads to the well-known torusshape observed in other clathrates. This demonstrates that the host−guest interactions are not merelyionic and that they significantly influence the guest-atom structure and dynamics. Comparison of derivedEinstein temperatures and bond distances for the three samples reveals that the compound with the highestaluminum content (flux growth) has the strongest host−guest interaction, even though it also has thelargest unit cell. Again, the specific properties of the guest atoms are not only determined by the clathratecage size but also by the subtle chemical interactions between the host structure and the guest. Thethermal conductivity is about 3 times smaller for the stoichiometric sample than for the Czochralski-pulled sample. Thus, control of the host-structure chemistry is not only a key to manipulating the electricalproperties of clathrates but also the thermal conductivity.
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