Study of Zn-substituted germanium clathrates as high performance thermoelectric materials assisted by first-principles electronic structure calculation

The thermoelectric properties of the clathrate compounds Ba ₈Zn_xGe_46-x were studied theoretically and experimentally. First, a first-principles electronic structure calculation was performed. The calculated result showed that Ba₈Zn₈Ge ₃₈ 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 E_g 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: E_g = 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 Ba₈Zn_xGe_46-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 (E_g = 0.9 eV) is wider than that of Ba₈Zn ₈Ge₃₈ (E_g = 0.4 eV), and that Ba ₈Zn₆Ga₄Ge₃₆ 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.