First-principles Study Of The Structural, Electronic And Thermodynamic Properties Of Typical Laves Phase In Mg Alloy

Laves phases with topologically close-packed structures are the most abundant intermetallic compound class. Laves phase exhibits a lot of exceptional functional properties, such as excellent corrosion and creep resistance, magnetic and electrical properties, which allows it to be served as magnetic materials, magneto-optical materials, hydrogen storage materials, etc. There exist three different types of Laves phase, including the cubic C15(MgCu2), hexagonal C14(MgZn2), and double-hexagonal C36(MgNi2) according to the difference of the particular stacking in the same four-layered structural units. Numerous experimental and theoretical studies have been performed to study the physical properties of MgCu2 and Mg Zn2 at room temperature and normal pressure, but the attempts under high pressure have not yet been reported. As known that the pressure can change the interaction potentials between atoms inside the solid material, then forming new target material with novel physical and chemical properties. Furthermore, the exiting of MgCu2 and MgZn2 precipitates in magnesium alloys which can improve their microstructures and mechanical properties. In this work, we present a detailed theoretical study of the structural, electronic and thermodynamic properties of MgCu2 and Mg Zn2 Laves phases under high pressure within the framework of DFT. In order to accelerate the achievement of Materials Genome Project, the present results can make help for the establishment of material database.Firstly, the ground-state properties of Mg Cu2 and MgZn2 are generated after geometry optimization. Then, we calculate the physical properties of MgCu2 and MgZn2 under different pressures. The detailed work has been given in the following:The optimized lattice constants of MgCu2 show better agreement with the experimental data and other theoretical values. Results show that the single-crystal elastic constants and mechanical moduli of MgCu2 increase monotonically with the increase of pressure. Moreover, C44 and its related shear deformation resistance increase comparatively slowly with increasing pressure. It means that the mechanical properties of MgCu2 can be enhanced under external pressure. Results of electronic structure show that the MgCu2 exhibits a metallic behavior due to many energy states crossing the Fermi level. In the low energy region, it shows the the TDOS are minaly dominated by Cu-d states, while the TDOS profile exibits a flat characteristic above the Fermi level because of the fully occupied d orbits of Cu. Interestingly, it is found that the TDOS exhibits a certain offset towards the low energy level with the applied pressure. Moreover, the highest peaks of the TDOS are broadened with the increase of pressure. Mulliken analysis shows the ionic character of MgCu2 can be improved under high pressure.Contrastive investigations have been carried out for the structural stabilities and mechanical moduli of MgCu2 and MgZn2 phases. The mechanical properties of Mg-based alloys can be improved by the formation of Mg Cu2 and MgZn2. Note that the lattice constants of MgZn2 decrease smoothly with the increasing pressure, and then become gentler with further increasing pressure. The polycrystalline elastic moduli of Mg Zn2 are determined by using the VRH approximation. It shows that the N(EF) decreases by about 39.62% from 0 GPa to 50 GPa, implying that the pressure is beneficial to its structural stability. In addition, the TDOS shifts towards the low energy region can be observed. Similarly, one can see the intensity of TDOS of MgZn2 below the Fermi level decreases with increasing pressure, which is similar to that of MgCu2. Furthermore, it shows that the TDOS split into some little peaks in the energy range around-8.0 eV. The intersity of TDOS also becomes weak with the applied pressure. These phenomena are highly associated with the delocalization of bonding atoms in MgZn2 crystal. We also conclude that the bonding characteristics in MgZn2 crystal are a combination of covalent, ionic and metallic nature.Finally, the thermodynamic quantities(Debye temperature(ΘD), heat capacity(CV and CP), thermal expansion coefficient(α) and Grüneisen parameter(γ)) of MgZn2 Laves phase under high pressure and temperature are calculated through the quasi-harmonic Debye model. Results show that the pressure effect on Debye temperature is more significant than that of temperature. The CV of MgZn2 increases rapidly when the temperature is below 300 K, which obeys the Debye T3 law. Above 300 K, the CV is independent much on temperature and then converges to the Dulong-Petit limit. The calculated CP is similar to CV in the low temperature range, and increases linearly at high temperature. It is found that the pressure effect on CP becomes less important with the increase of temperature. Besides, one can note that the temperature effect on α becomes less obviously at high temperature. These results have important meaning for understanding the physcial properties of typical Laves phases Mg Cu2 and MgZn2, also for guiding the future experimental work.