2009 Volume 50 Issue 3 Pages 631-639
The thermoelectric properties of the clathrate compounds Ba8ZnxGe46−x were studied theoretically and experimentally. First, a first-principles electronic structure calculation was performed. The calculated result showed that Ba8Zn8Ge38 is an intrinsic semiconductor with an indirect band gap, while Ba8Zn6Ge40 is an n-type degenerate semiconductor and Ba8Zn10Ge36 is a p-type degenerate semiconductor. A large x dependence for the band gap was found between x=6 and 8, i.e., the widths of band gap Eg at x=6, 8 and 10 were 0.83 eV, 0.40 eV, 0.35 eV, respectively. Thus, for a thermoelectric material with high performance at high temperatures, the band gap of the intrinsic semiconductor Ba8Zn8Ge38 is relatively small. On the other hand, it was found that the double substituted clathrate Ba8Zn6Ga4Ge36 is an intrinsic semiconductor with a relatively large band gap: Eg=0.69 eV. Second, we synthesized Zn substituted clathrate compounds by using the mechanical alloying and spark plasma sintering method, and measured the thermoelectric properties of the synthesized samples to show the concrete advantage of the Ba-Zn-Ga-Ge system. The experimental results showed that all of the Ba8ZnxGe46−x (x=6,8,10,12) samples were n-type semiconductors, and that some of the Zn clathrates (x=6∼10) had a reasonably good n-type thermoelectric ability. Moreover, it was confirmed that the band gap of Ba8Zn6Ga4Ge36 (Eg=0.9 eV) is wider than that of Ba8Zn8Ge38 (Eg=0.4 eV), and that Ba8Zn6Ga4Ge36 has better thermoelectric characteristics than Ba8Zn8Ge38 at high temperatures. Finally, the experimental thermoelectric properties were theoretically analyzed by using the results of the electronic structure calculation and good agreement was obtained.