Elastic Properties And Point Defect Of MgCaSi From Density Function Theory Calculations

The elastic properties can provide crucial information of mechanical properties of crystals and provide a basic criterion for the mechanical stability of crystal materials,and it is very helpful to study structural transformation and find new materials.Point defects is very valuable for understanding diffusion behavior and material’s ductility,strength,and creep resistance.In this paper,the elastic properties and point defect behaviors of MgCaSi are studied by using density functional theory.The main contents are as follows:Firstly,within density functional theory framework,the elastic and electronic properties of multi-performance ternary phase MgCaSi have been investigated in comparison with the mother phase Ca2Si.Energetic results and the calculated elastic constants show that MgCaSi is stable thermodynamically and mechanically.The derived elastic modulus of MgCaSi are higher than these of mother phase Ca2Si,demonstrating that hardness of MgCaSi has been favorably improved.Higher Debye temperature of MgCaSi indicates also the stronger interatomic interactions and better thermal conductivity.Although MgCaSi exhibits a smaller brittleness based on Pugh’s empirical formula,Poisson’s ratio and Cauchy pressure,the orthorhombic MgCaSi possesses lower anisotropy than Ca2Si from several criterions.To reveal the bonding nature of MgCaSi,electronic structures are further investigated.It is found that the strong Si-Si bond plays a significant role for structural stability and elastic property.Secondly,nine possible native point defects in MgCaSi have been studied by employing ab-initio calculations based on density functional theory.The complex chemical potential limits are first determined using two-dimension(△μMg,△μca)diagram,then the defect formation energies as a function of atomic chemical potential are gained.The energetic results show that under Mg-rich condition,the most favorable defect is Mgca rather than Mgsi,while CaMg is predominant compared to Casi under Ca-rich condition.The bonding energy is introduced to uncover the intrinsic feature of defect formation energy.The local geometric distortion around CaMg,Mgsi and Casi anti-site defects gradually increases due to the smaller atomic radii from Ca to Mg and Si,showing important role of the geometrical mismatch.The density of states(DOS)indicates that the higher stability of CaMg and Mgca originates from the smaller deviation of the Fermi level from the pseudo-gap.