Thermoelectric performance of large single crystal clathrate Ba8Ga16Ge30

A method for growing single crystals of the clathrate Ba₈Ga₁₆Ge₃₀ has been developed and a suitable specimen prepared. The crystal with a diameter of 7 mm and a length of 46 mm was cut into discs, which were analyzed with respect to chemical composition, crystal structure and thermoelectric properties. Typical disc samples were found to be n-doped with the Seebeck coefficient increasing from -50 μV/K at 300 K to -185 μV/K at 900 K. The thermal conductivity was found to be close to 2 W/mK and only slightly decreasing with increasing temperature. The resistivity of the sample increased from 0.65 mΩcm at 300 K to 1.8 mΩcm at 900 K. Combining these results the dimensionless thermoelectric figure-of-merit, ZT (= TS²/Rλ), was found to increase from ZT = 0.08 at room temperature to ZT > 1.0 at 900 K. Preliminary data indicates that ZT appears to further increase with temperature above 900 K. These results emphasize the position of the Ba₈Ga₁₆Ge₃₀ clathrate as being in the same range of thermoelectric performance as the current state of the art materials at the medium to high temperature range.