Synthesis and transport properties of ternary type-I Si clathrate   K8Al7Si39
  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. 
Shiva Kumar Singh, Takashi Mochiku, Soshi Ibuka, Yukihiro Isoda,   Akinori Hoshikawa, Toru Ishigaki, and Motoharu Imai
cond-mat.mtrl-sci