Synthesis and thermoelectric properties of p-type Ba8Ga16ZnxGe30−x type-I clathrates

Zn-doping Ba8Ga16ZnxGe30−x (x=2.4, 2.6, 2.8, 3.0, 3.2) clathrates with different Zn contents were synthesized by combining the solid-state reaction with the spark plasma sintering method. The effects of Zn content on thermoelectric properties were investigated. The results show that all specimens exhibit p-type conduction characteristics. The electrical conductivity, the room-temperature carrier mobility, and the thermal conductivity decrease with increasing Zn content for Zn-doped Ba8Ga16ZnxGe30−x compounds, while the room-temperature carrier concentration Np increases with increasing Zn content. The carrier scattering mechanism is mainly ionized impurity mechanism in the low-temperature range (T<200 K), and gradually changes to an acoustical mechanism with the rising temperature. In all p-type Ba8Ga16ZnxGe30−x compounds, Ba8Ga16Zn3.2Ge26.8 exhibits the highest Seebeck coefficient with the value of 250 μV/K at 300 K and 307 μV/K around 700 K, respectively. The maximum ZT value of 0.38 is obtained at 790 K for Ba8Ga16Zn3.0Ge27.0.