Synthesis and thermoelectric properties of type-VIII germanium clathrates Sr8AlxGayGe46−x−y

Nominal Sr8AlxGa16−xGe30 samples with x=6, 8, and 10 crystallized in the type-VIII clathrate structure (I4¯3m, No. 217), while the sample with x=4 crystallized in the type-I clathrate structure (Pm3¯n, No. 223). While a large number of the type-I thermoelectric clathrates exist, only three type-VIII clathrates of Ba8Ga16Sn30, Eu8Ga16Ge30, and Sr8AlxGa16−xSi30 had been synthesized before. The type-VIII Sr8AlxGayGe46−x−y samples (6≤x≤7 and 10≤y≤11) with various carrier concentrations were prepared to investigate their thermoelectric properties. They exhibited the temperature dependences of electrical conductivities and the Seebeck coefficients typical of n type degenerate semiconductors, which almost depended on their carrier concentrations systematically. A relatively large dimensionless figure-of-merit ZT of 0.56 at 800 K was obtained for the type-VIII Sr8Al6.3Ga10.3Ge29.4 sample with a carrier concentration of 3.0×1020 cm−3. This ZT value is comparable to that of 0.62 at 800 K for the type-I Sr8Ga16.5Ge29.5 clathrate. The type-VIII clathrate had a smaller effective mass, a higher mobility, and a higher lattice thermal conductivity than those of the type-I clathrate. The difference in transport properties between the type-I and type-VIII clathrates is also discussed.