Enhancing the thermoelectric power factor of Mg3Sb2with Sn doping on electronegative sites of Sb: Effects of reducing the electronegativity difference

The weighted mobility is a better descriptor of the optimal performance of electronic transport for thermoelectric materials. The smaller the electronegativity difference between two atoms, the more covalent the bonding character between these two atoms. A series of p-type Sn-doped Mg3Sb2 was prepared by mixing and grinding Mg, Sb, and wet chemically synthesized Sn powders, followed by cold pressing and heating in an evacuated-and-encapsulated ampoule at 500 °C. Both the electronic and thermal transport of p-Mg3Sb2-xSnx (x = 0, 0.02, 0.04, and 0.06) are investigated at temperatures ranging from 325 to 700 K. Sn doping on the electronegative Sb site lowered the electrical resistivity to 161 mΩ-cm at 325 K for x = 0.04, which is 91% lower than the pristine Mg3Sb2. The power factor is enhanced upon Sn doping as a result of increased weighted mobility peaking at x = 0.04. The highest power factor of the Mg3Sb1.96Sn0.04 alloy is 2.55 μW/cm-K2 having 16 times enhancement as compared to the pristine Mg3Sb2 at 700 K. As a result, Mg3Sb1.96Sn0.04 has a maximum zT of ∼0.30 at 700 K. The strategy of reducing the difference of electronegativity between electropositive cations and electronegative anions by partial replacement of Sn on electronegative Sb site is effective to increase the weighted mobility and hence enhance the power factor of p-Mg3Sb2.