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.