Preparation and thermoelectric properties of A(8)(II)B(16)(III)B(30)(IV) clathrate compounds

Polycrystalline samples of clathrate compounds Ba8Ga16Si30, Ba8Ga16Ge30, Ba 8Ga16Sn30, and Sr8Ga16Ge30 were prepared by direct melting and characterize d using X-ray powder diffraction and differential thermal analysis. The Ge- and Si-based clathrates melt congruently, whereas Ba8Ga16Sn30 melts incong ruently. At room temperature the Ge- and Si-based clathrates possess a mode rate negative Seebeck coefficient and a high electron concentration in the range of 7x10(20)-9x10(20) cm(-3) while Ba8Ga16Sn30 exhibits substantially lower electron concentration of 2.2x10(19) cm(-3). The Seebeck coefficient and electrical resistivity were measured over the range 100-870 K. The temp erature dependence of transport properties of the clathrates is typical for heavily doped semiconductors. The transport properties were analyzed using a standard semiconductor transport model. There is a good agreement betwee n the assumed model and experimental temperature dependence of the Seebeck coefficient in the extrinsic conductivity range for all studied clathrates apart from Ba8Ga16Ge30. The calculated effective masses of the clathrates r ange from 0.9 to 3 of the free electron mass. The estimated ZT values are 0 .7 for Ba8Ga16Ge30 at 700 K and 0.87 for Ba8Ga16Si30 at 870 K. The potentia l for thermoelectric applications of these materials is assessed. (C) 2000 American Institute of Physics. [S0021-8979(00)02911-X].