Crystal structure and thermoelectric properties of K _xBa _8-xZn _yGe _46-y clathrates

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.