Deformation mechanisms in high-efficiency thermoelectric layered Zintl compounds

Abstract

Recent experiments have uncovered n-type Zintl compounds including layered Mg₃Sb₂ with high thermoelectric efficiency. However, until now the mechanics of Mg₃Sb₂, which is important to understand for its widespread applications, has not been investigated. Here, we used density functional theory (DFT) to determine the deformation and failure mechanism of Mg₃Sb₂, and compared with isostructural CaMg₂Sb₂ and CaZn₂Sb₂ crystals. Mg₃Sb₂ is found to have a very low ideal shear strength of 1.95 GPa. The weakly ionic Mg–Sb bond, which links the Mg²⁺ sheet structure and the [Mg₂Sb₂]²⁻ substructure, breaks and creates pathways to slip between different [Mg₂Sb₂]²⁻ substructures under pure shear deformation in Mg₃Sb₂. The substitution of the Mg²⁺ sheet structure by a more electropositive Ca²⁺ leads to a much higher ideal shear strength of 4.07 GPa in isostructural CaMg₂Sb₂ compared with that in Mg₃Sb₂. The substitution of the [Mg₂Sb₂]²⁻ substructure by [Zn₂Sb₂]²⁻ in CaMg₂Sb₂ has little influence on the mechanical strength, leading to similar ideal shear strength for the CaZn₂Sb₂ and CaMg₂Sb₂. To enhance the mechanical strength of Mg₃Sb₂, we suggest that the weakly ionic Mg–Sb bond should be strengthened to improve the interaction between the Mg²⁺ sheet structure and the [Mg₂Sb₂]²⁻ substructure by appropriate doping strategies such as the partial substitution of Mg by more electropositive cations of Ca or Sr. These deformation modes are essential to understand the intrinsic mechanical process of this novel class of thermoelectric materials, which provides insightful guidance for designing high-performance layered Zintl compounds with improved strength and ductility.