Thermoelectric properties of type-I clathrate Na8Ga8Ge38

Abstract We investigated the use of Na as a guest atom for thermoelectric type-I Ge clathrates. Na is small for the framework cages; the guest Na atoms existed at off-center positions, or their atomic displacement parameters were relatively large. Despite its lightweight, there was a moderate reduction in thermal conductivity due to the rattling effect. The Na8Ga8Ge38 samples showed high thermoelectric power factors compared to Ba8Ga16Ge30. The former had larger effective masses and comparable carrier mobilities. The conduction-band bottom of Na8Ga8Ge38 consists of bands with a small inertial mass and bands with a large density-of-states mass. The s-orbitals of the Na guest atoms partly cause this desirable feature. A typical Na8Ga8Ge38 sample had lattice thermal conductivity of ∼20 mW cm−1K−1, the effective mass of ∼5me, and carrier mobility of ∼7 cm2V-1s-1 at room temperature along with a maximum dimensionless figure-of-merit ZT of 0.63 at 810 K. Na was thus found to be a useful guest atom for Ge clathrates.