Abstract
First principle calculations were performed to investigate the structural, mechanical, electronic properties, and thermodynamic properties of three binary Mg–B compounds under pressure, by using the first principle method. The results implied that the structural parameters and the mechanical properties of the Mg–B compounds without pressure are well matched with the obtainable theoretically simulated values and experimental data. The obtained pressure–volume and energy–volume revealed that the three Mg–B compounds were mechanically stable, and the volume variation decreases with an increase in the boron content. The shear and volume deformation resistance indicated that the elastic constant Cij and bulk modulus B increased when the pressure increased up to 40 GPa, and that MgB7 had the strongest capacity to resist shear and volume deformation at zero pressure, which indicated the highest hardness. Meanwhile, MgB4 exhibited a ductility transformation behaviour at 30 GPa, and MgB2 and MgB7 displayed a brittle nature under all the considered pressure conditions. The anisotropy of the three Mg–B compounds under pressure were arranged as follows: MgB4 > MgB2 > MgB7. Moreover, the total density of states varied slightly and decreased with an increase in the pressure. The Debye temperature ΘD of the Mg–B compounds gradually increased with an increase in the pressure and the boron content. The temperature and pressure dependence of the heat capacity and the thermal expansion coefficient α were both obtained on the basis of Debye model under increased pressure from 0 to 40 GPa and increased temperatures. This paper brings a convenient understanding of the magnesium–boron alloys.
Highlights
Magnesium boride alloys (MgB2, MgB4, and MgB7) as desirable compounds play an important role in many fields due to their remarkable conductivity, excellent ductility, and high h ardness[1,2,3]
The thermal expansion coefficient, Debye temperature, heat capacity, and other thermodynamic properties were theoretically studied for determining the pressure and temperature dependence of Mg–B compounds
The quasi-harmonic Debye model of the phonon density of states was implemented in this part of the study to investigate the thermodynamic behaviours of the Mg–B compounds under pressure, namely the heat capacity C v, Cp, the linear thermal expansion coefficient, and the Debye temperature ΘD of the Mg–B compounds
Summary
Magnesium boride alloys (MgB2, MgB4, and MgB7) as desirable compounds play an important role in many fields due to their remarkable conductivity, excellent ductility, and high h ardness[1,2,3]. The thermodynamic properties of Mg–B compounds and Al–Mg–B films were investigated by using ab initio calculations and CALPHAD m ethods[22,23]. The crystal structure, electronic properties, thermodynamic properties, and mechanical properties of Mg–B compounds at different pressure and temperature have not been studied. The above mentioned experimental studies have evidenced that the properties of Mg–B compounds can be calculated using DFT for establishing the trends of stability through the cohesive energies and the trends of charge transfers onto boron. In the current article, the structural, mechanism, electronic, and anisotropic properties of M gB2, MgB4, and MgB7 under pressure from 0 to 40 GPa were investigated by using DFT calculation. The thermal expansion coefficient, Debye temperature, heat capacity, and other thermodynamic properties were theoretically studied for determining the pressure and temperature dependence of Mg–B compounds
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