Antimony-induced heterogeneous microstructure of Mg2Si0.6Sn0.4 thermoelectric materials and their thermoelectric properties

Highlights: • Heterogeneous microstructured Mg₂Si_0.6Sn_0.4 was synthesized. • Antimony doping caused the formation of a Sn-rich grain boundary phase. • Coherent interfaces were formed between the Si-rich phase and the Sn-rich phase. • The bipolar effect was suppressed and the thermal conductivity was reduced. In order to achieve enhancements in thermoelectric efficiency, microstructures that can form numerous interfaces have been investigated intensively for controlling the transport of charge carriers and heat-carrying phonons. In this paper, we report the heterogeneous microstructure of Mg₂Si_0.6Sn_0.4 thermoelectric materials synthesized by a simple B₂O₃ encapsulation method and investigation of its influence on thermoelectric properties. The addition of Sb causes the evolution of a Sn-rich secondary phase and a heterogeneous microstructure consisting of Sn-deficient grains and a Sn-rich boundary phase, with coherent interfaces between them. The secondary phase induced by Sb doping suppressed the bipolar effect and reduced the thermal conductivity because of minority carrier blocking and phonon scattering at phase boundaries. However, high concentration of Sb in Sn-rich phase led to insufficient doping in Si-rich main phase and electron-hole compensation by Mg vacancies, resulting in decrease of the doping efficiency of Sb.