Polycrystalline samples of degenerate n-type K xBa 8-xZn yGe 46-y (y ∼ 8 - x/2) with the type-I clathrate structure (No. 223, Pm3̄n) were prepared by powder metallurgy to obtain a high-efficiency Ge-based clathrate. Their Zn atoms preferred to exist at the 6c site in the framework, and consequently, the samples with x around 4, such as K 4Ba 4Zn 6Ge 40, possessed highly ordered Zn/Ge atom frameworks whose 6c, 16i, and 24k sites were occupied almost solely by Zn, Ge, and Ge atoms, respectively. In spite of such ordered structures and small numbers of substituting Zn atoms, these samples exhibited carrier mobilities lower than those of Ba 8Zn 8Ge 38 and Ba 8Ga 16Ge 30. Band structure calculations implied that the combination of the rattler K and Ba atoms in the cages considerably modified the conduction band edge of the corresponding clathrates; such a modification is considered to strengthen alloy disorder scattering, which reduces carrier mobility. The maximum dimensionless figure-of-merit ZT was 0.51 at 1000 K for the K 2Ba 6Zn 7Ge 39 sample, which is similar to that of 0.50 at 900 K for the Ba 8Zn 8Ge 38 sample.
ASJC Scopus subject areas
- Physics and Astronomy(all)