Deformation mechanisms in high-efficiency thermoelectric layered Zintl compounds

Abstract

Recent experiments have uncovered n-type Zintl compounds including layered Mg_3Sb_2 with high thermoelectric efficiency. However, until now the mechanics of Mg_3Sb_2, 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 Mg_3Sb_2, and compared with isostructural CaMg_2Sb_2 and CaZn_2Sb_2 crystals. Mg_3Sb_2 is found to have a very low ideal shear strength of 1.95 GPa. The weakly ionic Mg–Sb bond, which links the Mg^(2+) sheet structure and the [Mg_2Sb_2]^(2−) substructure, breaks and creates pathways to slip between different [Mg_2Sb_2]^(2−) substructures under pure shear deformation in Mg_3Sb_2. The substitution of the Mg^(2+) sheet structure by a more electropositive Ca^(2+) leads to a much higher ideal shear strength of 4.07 GPa in isostructural CaMg_2Sb_2 compared with that in Mg_3Sb_2. The substitution of the [Mg_2Sb_2]^(2−) substructure by [Zn_2Sb_2]^(2−) in CaMg_2Sb_2 has little influence on the mechanical strength, leading to similar ideal shear strength for the CaZn_2Sb_2 and CaMg_2Sb_2. To enhance the mechanical strength of Mg_3Sb_2, we suggest that the weakly ionic Mg–Sb bond should be strengthened to improve the interaction between the Mg2+ sheet structure and the [Mg_2Sb_2]^(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.

Group Members

William A. Goddard III

Charles and Mary Ferkel Professor