First principles study of structural, electronic and elastic properties of Mg2Si polymorphs

The structural and the elastic properties of the Mg2Si polymorphs are calculated. The calculations are performed by using the plane-wave pseudo-potential method within the framework of first principles. The anti-fluorite structure, the anti-cotunnite structure and the Ni2In-type structure of Mg2Si can retain their mechanical stability in the pressure intervals 0-7 GPa,7.5-20. 2 GPa and 21.9-40 GPa, separately. The relationships between pressure and the elastic moduli ( elastic constant, bulk modulus, shear modulus, Young's modulus, Poisson ratio and anisotropy factor) are discussed. The electron density distribution, the density of states, the bond length and the Mulliken population of these polymorphs are systemically investigated. Our results show that the anti-fluorite Mg2Si is a semiconductor and the other two polymorphs are metallic materials. The interaction between Mg 2p, 3s and Si 3p plays a dominant role in the stability of the Mg2Si polymorphs. The strongest interactions in the anti-fluorite Mg2Si and the Ni2In-type Mg2Si are Mg-Mg and Mg-Si interactions, respectively. Our results are concordant with the experimental data and the previous results.