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Barium dynamics in noble-metal clathrates
Type I clathrates with noble transition metal Ba8TM6Ge40 (TM=Cu, Ag, and Au) have been investigated by inelastic neutron scattering, single crystal and synchrotron powder x-ray diffraction and DFT calculations to probe the structure and dynamics on the atomic scale. The DFT calculation assists in the assignment of the measured low energy phonon modes, and fair agreement is obtained between experiment and theory for the phonon density of states. The Cu compound shows good agreement, whereas the Ag and Au compounds show some discrepancies. Overall, the data show that the guest atom dynamics are similar in all samples. Physical property characterization was carried out on pellets pressed by spark plasma sintering. The transport properties show large differences with Ba8Cu6Ge40 being an n-type semiconductor, whereas Ba8Ag6Ge40 and Ba8Au6Ge40 are p-type semiconducting and metallic, respectively. The thermal conductivity was found to depend strongly on the charge carrier type. Thus, Ba8Cu6Ge40 has a crystal-like thermal conductivity while Ba8Ag6Ge40 and Ba8Au6Ge40 both have glasslike thermal conductivity. The samples exhibit thermoelectric figures of merit of about 0.06 at 400 K.