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

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 Mg_3.5Gd_0.04Sb₂ and Mg_3.5Ho_0.04Sb₂, 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.