Band structures and thermoelectric properties of the clathrates Ba8Ga16Ge30, Sr8Ga16Ge30, Ba8Ga16Si30, and Ba8In16Sn30

Density functional calculations in the generalized gradient approximation a re used to study the transport properties of the clathrates Ba8Ga16Ge30, Sr 8Ga16Ge30, Ba8Ga16Si30, and Ba8In16Sn30. The band structures of these clath rates indicate that they are all semiconductors. Seebeck coefficients, cond uctivities and Hall coefficients are calculated, to assess the effects of c arrier concentration on the quantity S(2)sigma/tau (where S is the Seebeck coefficient, sigma is the conductivity, and tau the electron relaxation tim e) which is proportional to the thermoelectric power factor. In each compou nd we find that both p- and n-doping will significantly enhance the thermoe lectric capabilities of these compounds. For p-doping, the power factors of all four clathrates are of comparable magnitude and have similar temperatu re dependence, while for n-doping we see significant variations from compou nd to compound. We estimate that room-temperature ZT values of 0.5 may be p ossible for optimally n-doped Sr8Ga16Ge30 or Ba8In16Sn30; at 800 K ZT value s as large as 1.7 may be possible. For single crystals of high quality, wit h substantially increased scattering times, the power factor of these mater ials will be significantly higher. Recent experiments are reviewed in the l ight of these calculations. (C) 2001 American Institute of Physics.