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

<jats:title>Abstract</jats:title> <jats:p>The maximum dimensionless figures-of-merit <jats:italic>ZT</jats:italic> <jats:sub>max</jats:sub> were 0.13 at 410 K for K<jats:sub>8</jats:sub>Al<jats:sub>8</jats:sub>Sn<jats:sub>38</jats:sub>, 0.25 at 440 K for K<jats:sub>8</jats:sub>Ga<jats:sub>8</jats:sub>Sn<jats:sub>38</jats:sub> and 0.21 at 420 K for K<jats:sub>8</jats:sub>In<jats:sub>8</jats:sub>Sn<jats:sub>38</jats:sub>, where their room temperature (RT) mobilities were 4.1 cm<jats:sup>2</jats:sup> V<jats:sup>−1</jats:sup> s<jats:sup>−1</jats:sup>, 31.0 cm<jats:sup>2</jats:sup> V<jats:sup>−1</jats:sup> s<jats:sup>−1</jats:sup> and 9.8 cm<jats:sup>2</jats:sup> V<jats:sup>−1</jats:sup> s<jats:sup>−1</jats:sup>, respectively, their RT effective masses were 4.3, 1.8 and 3.2 <jats:italic>m</jats:italic> <jats:sub>e</jats:sub>, respectively, and their RT lattice thermal conductivities were 12 mW cm<jats:sup>−1</jats:sup> K<jats:sup>−1</jats:sup>, 14 mW cm<jats:sup>−1</jats:sup> K<jats:sup>−1</jats:sup> and 13 mW cm<jats:sup>−1</jats:sup> K<jats:sup>−1</jats:sup>, 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 K<jats:sub>8</jats:sub>Al<jats:sub>8</jats:sub>Sn<jats:sub>38</jats:sub> sample was considered to be caused by strong alloy disorder scattering. Band structure calculations demonstrated that, in order of K<jats:sub>8</jats:sub>Ga<jats:sub>8</jats:sub>Sn<jats:sub>38</jats:sub>, K<jats:sub>8</jats:sub>In<jats:sub>8</jats:sub>Sn<jats:sub>38</jats:sub> and K<jats:sub>8</jats:sub>Al<jats:sub>8</jats:sub>Sn<jats:sub>38</jats:sub>, their conduction band edges were sharper and their band structures near the gaps were more similar to that of K<jats:sub>8</jats:sub>Sn<jats:sub>46</jats:sub>. These features were well consistent with their measured effective masses and mobilities.</jats:p>