First Principles Study on the Thermodynamic and Elastic Mechanical Stability of Mg₂X (X = Si,Ge) Intermetallics with (anti) Vacancy Point Defects

• Main Authors: Yuhong Zhao, Jinzhong Tian, Guoning Bai, Leting Zhang, Hua Hou
• Format: Article
• Language: English
• Published: MDPI AG 2020-03-01
• Series: Crystals
• Subjects: intermetallic compounds; thermodynamic stability; elastic mechanical stability; mg₂x (x = si, ge); vacancy point defects; first principle method
• Online Access: https://www.mdpi.com/2073-4352/10/3/234

In this paper, based on the density functional theory, through thermodynamic and mechanical stability criteria, the crystal cell model of intermetallic compounds with vacancy and anti-site point defects is constructed and the lattice constant, formation heat, binding energy, elastic constant, and elastic modulus of Mg₂X (X = Si, Ge) intermetallics with or without point defects are calculated. The results show that the difference in the atomic radius leads to the instability and distortion of crystal cells with point defects; Mg₂X are easier to form vacancy defects than anti-site defects on the X (X = Si, Ge) lattice site, and form anti-site defects on the Mg lattice site. Generally, the point defect is more likely to appear at the Mg position than at the Si or Ge position. Among the four kinds of point defects, the anti-site defect <inline-formula> <math display="inline"> <semantics> <mrow> <msub> <mi>x</mi> <mrow> <mi>M</mi> <mi>g</mi> </mrow> </msub> </mrow> </semantics> </math> </inline-formula> is the easiest to form. The structure of intermetallics without defects is more stable than that with defects, and the structure of the intermetallics with point defects at the Mg position is more stable than that at the Si/Ge position. The anti-site and vacancy defects will reduce the material&#8217;s resistance to volume deformation shear strain, and positive elastic deformation, and increase the mechanical instability of the elastic deformation of the material. Compared with the anti-site point defect, the void point defect can lead to the mechanical instability of the transverse deformation of the material and improve the plasticity of the material. The research in this paper is helpful for the analysis of the mechanical stability of the elastic deformation of Mg₂X (X = Si, Ge) intermetallics under the service condition that it is easy to produce vacancy and anti-site defects.