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A detailed investigation of the high-temperature thermoelectric properties of the ternary clathrate of type-I Ba8Zn x Ge46−x for the range 7.2 ⩽ x ⩽ 7.8 is presented. Electrical resistivity, Seebeck coefficient, thermal conductivity and Hall-effect measurements clearly demonstrate that the maximum ZT-value can be expected for the composition Ba8Zn7.7Ge38.3, which reaches ZT ∼ 0.46 at a temperature of 730 K. Furthermore, the influence on the thermoelectric properties was studied for partial substitution of Zn by one Ni-atom (Ba8Ni y Zn x Ge46−x−y with y nom = 1, and 6.5 ⩽ x ⩽ 7.2) or by three Ni-atoms per unit cell (Ba8Ni y Zn x Ge46−x−y with y nom = 3 and x = 1 to 3). Whereas a small amount of Ni increases the figure of merit ZT to 0.65 at 800 K, a higher Ni-content reduces the thermoelectric performance. Hall-effect measurements reveal that this improvement is essentially due to an increased charge carrier mobility.

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