Nearly isotropic transport properties in anisotropically structured n-type single-crystalline Mg3Sb2

Abstract

As a top candidate for competing with the conventional n-Bi2Te3 thermoelectrics, Mg3Sb2-based materials have attracted increasing attentions for low-grade (<300 °C) waste heat recovery applications, due to the high thermoelectric performance, low cost, abundance and nontoxicity. Because of its anisotropic crystal structure as that of Bi2Te3, possible property anisotropy and the resultant similar requirement of texturing for preferential performance remain not entirely clear Mg3Sb2-thermoelectrics at working temperatures. This motivates the current work to focus on the transport-property anisotropy of n-type Mg3Sb2, centimeter-sized single crystals of which are successfully grown by a flux-assisted vertical Bridgman technique with a post-annealing under a Mg vapor pressure. This enables a revelation of nearly isotropic transport properties in this anisotropically structured material, guaranteeing reasonably high performances in polycrystalline materials achievable by cheap and scalable processing approaches such as powder-metallurgy. In addition, this work uncovers the strong lattice anharmonicity and large band degeneracy are respectively the origins for the low lattice thermal conductivity and superior electronic performance.