Tuning of band gap and thermoelectric properties of type-I clathrate Ba8NixZnyGe46−x−y−zSnz

• Quinary clathrate-I compounds with n-type electrical conductivity.
• Tuning of the band gap via Ni-substitution.
• High thermoelectric performance (ZTmax = 0.9 at 830 K).

A detailed investigation of the influence of different Ni/Zn-ratios on the thermoelectric behavior of the quinary clathrate-I Ba8NixZnyGe46−x−y−zSnz is provided. All samples crystallize in the clathrate-I structure type (space group Pm 3¯n) and exhibit n-type electrical conductivity. Varying the Ni-content, the band gap can be tuned in a wide range and hence the maximum in the thermoelectric figure of merit, ZT, can be shifted in a large temperature range from 530 to 830 K. The highest ZT-value of 0.9 at 830 K was observed for the Ni-poor composition Ba8Ni0.22Zn7.22Ge37.12Sn1.44.