Preparation and thermoelectric properties of sintered n-type K8M8Sn38 (M = Al, Ga and In) with the type-I clathrate structure

The maximum dimensionless figures-of-merit ZTmax were 0.13 at 410 K for K8Al8Sn38, 0.25 at 440 K for K8Ga8Sn38 and 0.21 at 420 K for K8In8Sn38, where their room temperature (RT) mobilities were 4.1 cm2 V-1 s-1, 31.0 cm2 V-1 s-1 and 9.8 cm2 V-1 s-1, respectively, their RT effective masses were 4.3, 1.8 and 3.2 me, respectively, and their RT lattice thermal conductivities were 12 mW cm-1 K-1, 14 mW cm-1 K-1 and 13 mW cm-1 K-1, respectively. The analysis of the measured transport properties suggested that these samples suffered from grain boundary scattering at lower temperatures and from alloy disorder scattering and acoustic phonon scattering at higher temperatures. In particular, such a low mobility of the K8Al8Sn38 sample was considered to be caused by strong alloy disorder scattering. Band structure calculations demonstrated that, in order of K8Ga8Sn38, K8In8Sn38 and K8Al8Sn38, their conduction band edges were sharper and their band structures near the gaps were more similar to that of K8Sn46. These features were well consistent with their measured effective masses and mobilities.