Type VIII Si based clathrates: prospects for a giant thermoelectric power factor.

Although clathrate materials are known for their small thermal conductivity, they have not shown a large thermoelectric power factor so far. We present the band structures of type VIII Si, Ge, and Sn clathrates as well as the alkali and alkaline-earth intercalated type VIII Si clathrates. Our calculations revealed that this group of materials has potentially large power factors due to the existence of a large number of carrier pockets near their band edges. In particular, we calculated the charge carrier transport properties of Si46-VIII both for n-type and p-type materials. The exceptionally high multi-valley band structure of Si46-VIII near the Fermi energy due to the high crystallographic symmetry resulted in a giant power factor in this material. It was shown that the intercalation of Si46-VIII with alkali and alkaline-earth guest atoms shifts the Fermi energy close to the conduction band edge and, except for Be8Si46 and Mg8Si46, they weakly influence the band structure of Si46. Among these clathrate systems, Ca8Si46, Sr8Si46, and Ba8Si46 showed negative formation energy, which should facilitate their synthesis. Our results imply that the intercalation affects the conduction band of Si46-VIII more than its valence band. Also, interestingly, the type VIII clathrates of Si46 and its derivatives (except Be8Si46 and Mg8Si46), Sn46, and Ge46 all have 26 carrier pockets near their valence band edge. Among the different derivatives of Si46-VIII, Rb8Si46 and Ba8Si46 have the highest number of electron pockets near their band edges. The thermoelectric power factor was predicted using a multiband Boltzmann transport equation linked with parameters extracted from density functional calculations. It was shown that both the increment of charge mobility and the existence of multiple band extrema contribute to the enhancement of the thermoelectric power factor considerably. Such a large power factor along with their inherently low thermal conductivity can make this group of clathrates promising thermoelectric materials.