Crystal Growth and Thermoelectric Properties of Zintl Phase Mg3X2 (X=Sb, Bi) Based Materials: a Review

Zintl phase Mg3X2 (X=Sb, Bi) based thermoelectric materials have attracted much attention because of their non-toxic, low cost and high performance. Compared with polycrystalline materials, the Mg3X2 crystals are of great value in revealing material's intrinsic and anisotropic thermoelectric properties, as well as providing effective strategies for enhancing electrical and thermal transport properties. Therefore, the recent progress of single crystal growth and thermoelectric properties for Mg3X2 crystals are systematically summarizes in this paper. Due to the volatility and causticity of Mg element, several different methods such as slow cooling method, directional solidification method, flux method, and flux Bridgman method are widely used for synthesizing Mg3X2 crystals, in which the flux Bridgman method is more competitive to prepare large size bulk crystals. Researchers found that both n-type and p-type Mg3Sb2 crystals show an anisotropy thermoelectric transport property. The crystal growth rate, the concentration of self-doped Mg element, the concentration of impurity doping or alloying elements have a great impact on both electrical and thermal transport properties for Mg3Sb2 crystals. So far, the p-type and n-type Mg3Sb2 crystals with ZT value of 0.68 and 0.82 are achieved, respectively. This paper reviews the recent progress of growth and thermoelectrics properties of Zintl phase Mg3X2-based crystals, revealing that the flux Bridgman method is the most effective method to produce large-sized Mg3X2-based crystals. Tuning chemical composition of Mg3X2-based crystal by doping and forming solid solution for optimal carrier concentration and band structure engineering is expected to further improve the thermoelectric performance of Mg3X2-based crystal. The above-mentioned growth method and research strategies provide a significant guidance for the in-depth understanding of the Mg3X2-based crystal in the future.