First-principles calculation on the structure stability, elastic properties and electronic structure of P-doped Mg₂Si

In this study, the effect on the structure stability, elastic properties and electronic structure of P-doped Mg₂Si were studied by the first-principles pseudopotential plane wave method based on density functional theory. The lattice constants, formation enthalpy, cohesive energy, elastic constants, and elastic moduli of Mg₂Si, Mg₇Si₄P, Mg₈Si₃P and Mg₈Si₄P were calculated, and the electronic structure analysis was also performed. The occupation tendency, structural stability, bonding characteristics, orbital hybridization and the change of conductivity of doping P atoms in the matrix were further investigated. Among them, the research results of formation enthalpy, cohesive energy and elastic constant show that Mg₂Si, Mg₈Si₃P and Mg₈Si₄P can all exist stably in the system, and the crystal structure of Mg₇Si₄P can not exist stably. P atoms doping into the Mg₂Si lattice tend to occupy Si atoms position preferentially. The results of elastic modulus study show that Mg₂Si and Mg₈Si₄P are brittle phase and Mg₈Si₃P is ductile phase. The plasticity and toughness of Mg₂Si alloy system are improved by doping P atoms. The electronic structure analysis shows that the method of doping P atoms changes the orbital hybridization and bonding characteristics of the system. The Mg-P and Si-P covalent bond formed by Mg₈Si₃P and Mg₈Si₄P increase the structure stability. The energy band structure analysis also show reduction of the band gap from 0.224 to 0.184 eV for Mg₂Si with P dopants at the substitutional Si-sites and the band gap closure in the system with interstitial P-impurities. It enhances the metallic property of the material, and Mg₈Si₄P phase also transform from its semiconducting to metallic state. Consequently, this method both increases the carrier concentration and reduces the energy of free electron transition. The conductivity of the Mg₂Si alloy system will be improve. (paper)