Improved Thermoelectric Properties of N-Type Mg3Sb2 through Cation-Site Doping with Gd or Ho.

Abstract

The success of n-type doping has attracted strong research interest for exploring effective n-type dopants for Mg3Sb2 thermoelectrics. Herein, we experimentally study Gd and Ho as n-type dopants for Mg3Sb2 thermoelectrics. The synthesis, structural characterization, and thermoelectric properties of Gd-doped, Ho-doped, (Gd, Te)-codoped, and (Ho, Te)-codoped Mg3Sb2 samples are reported. It is found that Gd and Ho are effective n-type cation-site dopants showing a higher doping efficiency as well as a superior carrier concentration in comparison with anion-site doping with Te, consistent with the previous theoretical prediction. For n-type Mg3Sb2 samples doped with Gd or Ho, optimal thermoelectric figure of merit zT values of ∼1.26 and ∼0.94 at 725 K are obtained, respectively, in Mg3.5Gd0.04Sb2 and Mg3.5Ho0.04Sb2, which are superior to many reported Te-doped Mg3Sb2 without alloying with Mg3Bi2. By codoping with Gd (or Ho) and Te, reduced thermal conductivity and enhanced power factor values are achieved at high temperatures, which results in enhanced peak zT values well above unity at 725 K. This work reveals Gd and Ho as effective n-type dopants for Mg3Sb2 thermoelectric materials.