High thermoelectric performance of type-III clathrate compounds of the Ba–Ge–Ga system

The thermoelectric properties of type-III clathrate compounds, Ba24GaXGe100−X, have been investigated as a function of Ga content and temperature. The substitution of Ga atoms for Ge atoms leads to a decrease of carrier (electron) concentration. Electrical conduction is of n-type for all clathrate compounds investigated and the values of electrical resistivity and Seebeck coefficient increase with increasing Ga content and temperature. Both electronic and lattice thermal conductivities decrease with an increase in Ga content because of the decreased carrier concentration and the increased extent of the rattling motion of Ba atoms encapsulated in open dodecahedrons, respectively. A very high thermoelectric figure of merit (ZT) value of 1.25 is obtained at 670 °C when X = 15.