Synthesis and thermoelectric properties of the quaternary type-I Si clathrates K8-δGaxAlySi46-x-y

The effects of substituting Ga atoms with Al on the thermoelectric properties of the ternary Si clathrate K8Ga8Si38 were assessed over the temperature range of 10-700 K. For this purpose, the series of quaternary Si clathrates K8-δGaxAlySi46-x-y (x + y ≈ 8; 0 ≤ y < 8) was synthesized by heating mixtures of K, Ga, Al and Si at temperatures between 1223 and 1353 K. The lattice constant of K8-δGaxSi46-x was found to increase with the Al content, y, and Al substitution also significantly changed the transport properties of the clathrate, even though Al and Ga are isovalent. Electrical resistivity measurements showed that the conduction mechanism changed from variable range hopping conduction to metallic conduction with increasing y. The Seebeck coefficient, S, was negative for all of the samples, indicating that the dominant carriers were electrons, while the absolute value of S decreased with y. All specimens had approximately the same value of thermal conductivity, κ, on the order of 1.4 W/Km at 300 K, except for K7.7Al7.5Si38.8. K7.5Ga4.8Al3.0Si38.7 exhibited the largest power factor, and therefore the highest thermoelectric dimensionless figure of merit, ZT, with a value of 0.066 at 550 K. This value is approximately six times that of the ternary Si clathrate K7.4Ga7.7Si38.9.