Thermoelectric power generation has attracted much attention in times of energy shortage. Direct conversion from waste heat to electrical energy is feasible by using a thermoelectric module made of p- and n-type materials . Efficient thermoelectric materials need to have simultaneously high electrical conductivity as a crystalline metal (electron-crystal) and low thermal conductivity as a glass (phonon glass). I report on our recent studies of the structural, electronic and vibrational behaviors of a dimorphic clathrate Ba8Ga16Sn30 (BGS) possessing type-I and type-VIII structures . In type-I BGS, the Ba atom in the tetrakaidecahedron occupies the off-center sites which are 0.43 Å away from the center. This displacement results from the mismatch between the guest ion size and the host cage radius. The symmetry-forbidden mode in the Raman spectrum gives direct evidence that the Ba atoms are rotating among the off-center sites. This mode has a characteristic energy of 20 K whose energy is lowest among intermetallic type-I clathrates. The optical conductivity peak of this rattling mode splits into two peaks and shows non-Boltzmann broadening with decreasing temperature below 100 K. The glasslike thermal conductivity with a plateau at 4-12 K is therefore ascribed to the strong interaction of acoustic phonons with the low-energy off-center rattling mode. The type-I phase of BGS transforms into type-VIII phase above 739 K on heating, but this transformation is not reversible. Therefore, we have made our efforts to improve the thermoelectric properties of type-VIII BGS. The Ba guests are nearly on center of the distorted dodecahedron. Nevertheless, the thermal conductivity is as low as 0.7W/Km at T > 300 K. In this compound, the carrier type and its density are tunable by controlling the initial amount of flux, Ga or Sn. The dimension less thermoelectric figure of merit ZT for the optimally doped single crystals has the maximum values of 1.0 and 0.9 at 450 K for p- and n-type samples, respectively. By substituting Al, Zn, Ge, In, and Sb for the Ga and Sn atoms, we have attempted to enhance the ZT value. As a result, the value of n-type is increased to 1.45 at 520 K, where the conventional materials based on Bi-Te and Pb-Te had a valley in ZT. Thus, the BGS without toxic elements is a promising thermoelectric material at temperatures 400-600 K. I will present our preliminary assessment of thermoelectric efficiency of modules made of p- and n-type BGS. This work was supported by a NEDO grant no. 09002139-0 and Grant-in-Aid for Scientific Research from MEXT of Japan, grants no. 19051011 and no. 20102004.  D. M. Rowe (Ed), Thermoelectrics and its energy harvesting, CRC Press, Taylor & Frances Group, Boca Raton, FL, 2012.  T. Takabatake, in: K. Koumoto and T. Mori (Eds.), Thermoelectric Nanomaterials, Springer Series in Materials Science Vol. 182, Springer, Heidelberg, 2013, p. 33.
- 1980-1981 : Postdoctoral Research Fellow of Japan Society for Promotion of Science, Institute for Chemical Research, Kyoto University.
- 1981-1983 : Research Fellow of the Alexander von Humboldt Foundation, Germany Institute für Festkörperforschung, KFA Jülich, Germany.
- 1983-1984 : Research Fellow, Institute für Experimental Physik IV, Ruhr-Universität Bochum, Germany.
- Research Associate
- 1984-1988 : Institute for Solid State Physics, Tokyo University Associate Professor
- 1988-1995 : Faculty of Integrated Arts and Sciences, Hiroshima University
Awards and Honors
Memberships and Honors
- Member of the Physical Society of Japan
- Member of the Magnetics Society of Japan
- Member of the Thermoelectric Society of Japan
- 2000 winner of Outstanding Scientific Accomplishment Award for the Research in Metal Physics
- 2000. 04 – 2003. 03 : $690kU.S., NEDO Grant for International Joint Research, Japan, “Novel thermoelectric functions of rare-earth based compounds with valence instabilities”
- 2001. 04 – 2006. 03 : $5,900kU.S., Grant in Aid for Scientific Research for COE Project, MEXT, Japan, “Novel functional materials with multinary freedoms”
- 2006. 04 – 2010. 03 : $325kU.S., Grant in Aid for Scientific Research (A), MEXT, Japan, “Thermoelectrics of clathrates with rare-earth- and alkaline-earth guests”
- 2008. 11 – 2013. 03 : $1,320kU.S., Grant in Aid for Scientific Research on Innovative Areas, MEXT, Japan, “Search for new rattling materials, crystal growth, and study of electron-phonon properties”
- 2009. 07 – 2012. 03 : $510kU.S., NEDO Grant for Novel Materials by Nano-Structuring, Japan, “Development of high-performance thermoelectric materials by controlling nano-structure of caged compounds”
- T. Takabatake, in Thermoelectric Nanomaterials, ed by K. Koumoto, T. Mori (Springer, Heidelberg, 2013), pp. 33-49.
- Y. X, Chen, B. L. Du, Y. Saiga, K. Kajisa, T. Takabatake, Crystal growth and thermoelectric properties of type-VIII clathrate Ba8Ga15.9Sn30.1-xGex with p-type carriers, J. Phys. D: Appl. Phys. 46, 205302/1-6, 2013.
- Y. Saiga, B. Du, S. K. Deng, K. Kajisa, T. Takabatake, Thermoelectric properties of type-VIII clathrate Ba8Ga16Sn30 doped with Cu, J. Alloys and Compd., 537, 303-307, 2012.
- S. Kimura, T. Iizuka, H. Miyazaki, A. Irizawa, Y. Muro, T. Takabatake, Electronic-structure-driven magnetic ordering in a Kondo semiconductor CeOs2Al10, Phys. Rev. Lett. 106, 056404/1-4, 2011.
- T. Onimaru, K. T. Matsuomoto, Y. F. Inoue, K. Umeo, T. Sakakibara, Y. Karaki, M. Kubota, T. Takabatake, Antiferroquadrupolar ordering in a Pr-based superconductor PrIr2Zn20, Phys. Rev. Lett. 106, 177001/1-4, 2011.
- T. Mori, K. Iwamoto, S. Kushibiki, H. Honda, H. Matsumoto, N. Toyota, M. A. Avila, K. Suekuni, T. Takabatake, Optical conductivity spectral anomalies in the off-center rattling system -Ba8Ga16Sn30, Phys. Rev. Lett. 106, 015501/1-4, 2011.
- K. Suekuni, Y. Takasu, T. Hasegawa, N. Ogita, M. Udagawa, M. A. Avila, T. Takabatake, Off-center modes and glasslike thermal conductivity in the type-I clathrate Ba8Ga16Sn30, Phys. Rev. B 81, 205207 (1-5), 2010