Elastic And Electronic Properties Of Ca_（2-x）Mg_xSn（X=0,1,2） From First-Principles Calculations

Mg-based alloys have attracted considerable attention for the aerospace and automotive applications due to the advantages of low density, high specific strength and good cast ability. However, the applications are still limited because of the lower strength, inferior fatigue and creep-resistance at elevated temperature. Hence, much effort has been devoted to improve the mechanical properties of magnesium alloys. It is shown that the mechanical properties of magnesium alloys could be improved remarkably by the alloying effect. Al-free Mg alloys have been currently attracting attention as a next generation of commercial Mg alloys, and Mg-Sn-Ca is the important systems among the Al-free Mg alloys. So we have carried out first-principles calculations within the density functional theory to investigate the elastic and electronic properties of the Ca(2-x)MgxSn (x=0,1,2) in this work.The optimized and calculated atomic internal coordination and lattice constants are in good agreement with experimental values. The negative formation enthalpies show that these three phases are thermodynamically stable. The Ca2Sn phase is the most stable, following by CaMgSn and Mg2Sn. The calculated elastic constants obey the Born criteria of mechanical stability. When the four Cal in Ca2Sn is substituted by Mg, the elastic constants, elastic moduli, the stability against shear, brittleness and Debye temperature of CaMgSn are increased, while the degree of elastic anisotropy is decreased. For the cubic structure Mg2Sn, these properties are very close to CaMgSn. In order to describe the interaction between atoms the density of states (DOS), band structures and charge density of Ca(2-x)MgxSn (x=0,1,2) are studied. The DOS and band structures show that Ca2Sn is a semiconductor with the direct band gap value of0.068eV, while CaMgSn and Mg2Sn are conductors without band gap. The pseudogap indicates the presence of covalent bonding in CaMgSn and Mg2Sn. In the charge density Ca2Sn mainly has the Ca-Sn bonding which exhibits a covalent bonding with ionic characteristics. When the four Cal atoms in Ca2Sn are substituted by Mg, CaMgSn has a strong covalent bonding Sn-Sn. And Mg2Sn also has the strong covalent bonding Sn-Sn as in CaMgSn, which is consistent with the DOS discussion about the covalent bonding. Therefore, the Sn-Sn covalent bonding plays an important role in improving the elastic behavior of Ca(2-x)MgxSn (x=0,1,2).