Mechanical Properties Of Mg₂X (X= Si, Ge, Sn , Pb) From The First-principles Calculations

Because of shorting of traditional fossil energy, global warming and environmental pollution become more and more serious. Thermo-electric technology as one way to solve the energy crisis attracts widespread attention. Thermo-electric technology is to achieve a direct conversion between thermal and electrical energy with solid carrier inside a material. Thermo-electric materials can be used not only in some special areas (such as space exploration area) for power generation and refrigeration, but also in recycling waste of heat. At present, thermo-electrical materials are mainly TeBi (Pb) and their alloys, the Si-Ge alloys are also an important one. The anti-fluorite structured Mg2X (X=Si, Ge, Sn and Pb) compounds are also the typical thermo-electrical materials and have attracted broadly attention due to their large abundance resources, without expensive rare earth elements and toxic Pb element. However, the low efficient thermo-electric transformation and inferior mechanical properties has severely limited its wider application for a long time. Recently tremendous development in the theoretical simulation of the materials mechanical properties provides a wider range of opportunity to explore the thermoelectric materials owning excellent mechanical properties and adaptation of industrial production.In present study, first-principles calculations within generalized gradient approximation have been performed to investigate elastic properties and ideal strengths of anti-?uorite structured Mg2X (X=Si, Ge, Sn and Pb) compounds. First, the optimized structural parameters and elastic constant were in good agreement with the available experimental data. The calculated mechanical properties and ideal strength showed that with increasing of atomic number of X element, the bulk modulus, shear modulus and Young's modulus became weak, and all of Mg2X compounds exhibited brittleness. The present calculations further showed that the ideal tensile strengths of Mg2X occurred in the [111] tensile direction and ideal shear strengths occurred in the(111)[112]shear direction. Their ideal strengths become weak with the increasing of atomic number ofⅣgroup element, which are good consistent with the mechanical properties obtained by small strain situation. Then the inherent mechanisms of mechanical properties were discussed from the structural evolution, electronic structures and so on.Then, first-principles calculations were also used to investigate the mechanical properties of pure magnesium at the elastic limit and at large strain. The calculated lattice constants and elastic constants were in good agreement with the available experimental data. The obtained results showed that pure magnesium exhibited ductility. The ideal strengths of pure magnesium at large strains along typically crystallographic directions were calculated. The results indicated that the ideal tensile strength of hcp-Mg occurred in the [2023] direction and its value is 1.410 GPa. The ideal shear strength occurred in the (101 0 )[1 210 ] shear direction and corresponding value is 1.027 GPa. The calculated results along the (0001)[1 210 ] shear direction agreed well with previous investigations. Because ideal shear strength is larger than ideal tensile strength, shear fracture would take place easily. Lastly, the evolutions of charge densities provided vivid insights into the underlying mechanism for mechanical properties of magnesium at large strains.