Synthesis and thermoelectric properties of quaternary Si clathrate K8−δGaxZnySi46−x−y

Abstract

The thermoelectric properties of quaternary Si clathrates, K8−δGaxZnySi46−x−y, were investigated at temperatures from 10 to 320 K to determine the effects of Ga atom substitution by an aliovalent atom, Zn, on the thermoelectric properties of the ternary Si clathrate K8Ga8Si38. Only a small amount of Ga atoms could be substituted by Zn, 0 ≤ y ≤ 0.8, and the lattice constant of K8−δGaxZnySi46−x decreased with an increase in the Zn content, y. Zn substitution significantly changed the transport properties of the clathrate. Measurements of electrical resistivity and the Seebeck coefficient, S, revealed that the conduction mechanism changed from variable range hopping to metallic conduction with an increase of y. S for all of the samples was negative, which indicates that the dominant carriers were electrons, and the absolute value of S decreased as y increased. All the specimens had approximately the same thermal conductivity of ca. 1.6 W K−1 m−1 at 300 K. K7.6Ga5.8Zn0.8Si39.6 exhibited the largest power factor, and therefore the highest thermoelectric dimensionless figure of merit ZT, of 0.0096 at 320 K, which is approximately twice that for the ternary Si clathrate K7.4Ga7.7Si38.9. Thus, Zn substitution enhances the ZT of K7.4Ga7.7Si38.9.