A ternary type-I Si clathrate, K8AlxSi46-x, which is a candidate functional material composed of abundant non-toxic elements, was synthesized and its transport properties were investigated at temperatures ranging from 10 to 320 K. The synthesized compound is confirmed to be the ternary type-I Si clathrate K8Al7Si39 with a lattice parameter of a = 10.442 A using neutron powder diffractometry and inductively coupled plasma optical emission spectrometry. Electrical resistivity and Hall coefficient measurements revealed that K8Al7Si39 is a metal with electrons as the dominant carriers at a density of approximately 1x10^27 /m3. The value of Seebeck coefficient for K8Al7Si39 is negative and its absolute value increases with the temperature. The temperature dependence of the thermal conductivity is similar to that for a crystalline solid. The dimensionless figure of merit is approximately 0.01 at 300 K, which is comparable to that for other ternary Si clathrates.
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