Intermetallic clathrates are the representative of nano-cage structured material showing low thermal conductivity . A dimorphic clathrate Ba8Ga16Sn30possesses type-I and type-VIII structures. The type-I phase transforms to the type-VIII phase on heating above 739 K. The type-I compound displays the glasslike thermal conductivity with a plateau at 4-12 K, which is ascribed to the strong interaction of acoustic phonons with the low-energy off-center rattling mode. In the type-VIII compound, the Ba guest is vibrating nearly on center of the distorted dodecahedron. Nevertheless, the thermal conductivity is as low as 0.7W/Km for T> 300 K. The carrier type and its density in single crystals can be tuned by controlling the initial amount of flux, Ga or Sn. Further optimization of carrier density by alloying has led to high ZT of 1.0 and 1.45 for p- and n-type, respectively, at around 500 K, where the conventional materials based on Bi-Te and Pb-Te had a valley in ZT. By assembling p- and n-type legs made of Ba8Ga16Sn30, a module was prepared with the segments of Bi-Te legs. The conversion efficiency reached 7.5% at the temperature difference of 390 K.
 T. Takabatake et al., Rev. Mod. Phys. 86, 669-716 (2014).