Large valley degeneracy and high thermoelectric performance in p-type Ba8Cu6Ge40-based clathrates

We demonstrate both theoretically and experimentally the high thermoelectric performance of p-type Ba8Cu6Ge40. Density functional theory calculations for Ba8Cu6Ge40 find that the valence band maximum consists of 12-fold degenerated valleys with light band effective masses, indicative of excellent electronic properties. It is also indicated that changing the Cu/Ge ratio is effective in controlling the carrier type. Motivated by these calculation results, a series of Ba8Cu6−xGe40+x samples with different Ge substitution amounts x are fabricated and the transport properties are characterized. The carrier type is effectively controlled with x, and the p-type Ba8Cu5.7Ge40.3 sample shows a high power factor of ∼1 mW/mK2, much higher than the previously reported values of n-type samples. These results emphasize the importance of the descriptor-based investigation into the electronic structures of clathrate thermoelectric materials.