The effects of In isoelectronic substitution for Ga on the thermoelectric properties of Sr8Ga16-xInxGe30 type-I clathrates

n-type Sr8Ga16-xInxGe30 ( x = 0, 0.5, 1.0, 1.5) clathrates have been synthesized by combining melting reaction with the spark plasma sintering method. The effects of isoelectronic substitution of In for Ga on thermoelectric properties have been investigated. The results show that the solubility limit of In in the Sr - Ga - Ge system is less than 1.5 when it is expressed by the formula of Sr8Ga16-xInxGe30. With increasing In content x, the cell parameter and the electrical conductivity of the Sr8Ga16-xInxGe30 compound increase, while the Seebeck coefficient decreases. The isoelectronic substitution of In for Ga leads to a small variation of the room- temperature carrier concentration, an increase in the room- temperature carrier mobility as well as a remarkable decrease in lattice thermal conductivity. As a result, the maximum dimensionless figure of merit ZT of 0.72 is obtained at 800K for Sr8Ga15.5In0.5Ge30. Compared with that of ternary alloy Sr8Ga16Ge30, it increases by 38% at the same temperature.