Improved figure of merit (z) at low temperatures for superior thermoelectric cooling in Mg 3 (Bi,Sb) 2 .

The low-temperature thermoelectric performance of Bi-rich n-type Mg 3 (Bi,Sb) 2 was limited by the electron transport scattering at grain boundaries, while removing grain boundaries and bulk crystal growth of Mg-based Zintl phases are challenging due to the volatilities of elemental reactants and their severe corrosions to crucibles at elevated temperatures. Herein, for the first time, we reported a facile growth of coarse-grained Mg 3 Bi 2-x Sb x crystals with an average grain size of ~800 μm, leading to a high carrier mobility of 210 cm 2  · V -1  · s -1 and a high z of 2.9 × 10 -3  K -1 at 300 K. A [Formula: see text]T of 68 K at T h of 300 K, and a power generation efficiency of 5.8% below 450 K have been demonstrated for Mg 3 Bi 1.5 Sb 0.5 - and Mg 3 Bi 1.25 Sb 0.75 -based thermoelectric modules, respectively, which represent the cutting-edge advances in the near-room temperature thermoelectrics. In addition, the developed grain growth approach can be potentially extended to broad Zintl phases and other Mg-based alloys and compounds.