Point Defects In Mg₂Ca And MgZn₂ Laves Phases:A First-Principles Study

Point defect is one of the most important crystal defects. It is a local disorder of the crystal lattice and only affects a few atoms around point defect. However, it is difficult or even impossible to perform the experimental study on point defect. Based on the density functional theory, the first-principles calculation has been successfully simulated the work that experiments can not carry out. In this study, the point defect of magnesium alloys Laves phases Mg2Ca and MgZn2 has been studied and discussed deeply from first-principles calculation based on the density functional theory (DFT), the main contents of this study are as follows:Firstly, the native point defects in C14 Mg2Ca Laves phase are studied from the first-principles density functional theory calculations within GGA approximation. The defect formation energies indicate that anti-site defects are energetically favored over vacancies. Under Mg-rich and even general Ca-rich condition, defect MgCa of Mg anti-site on Ca sublattice is favorable owing to the lowest formation energy. The CaMg2 defect of Ca anti-site on Mg2 sublattice is also likely dominant only under extreme Ca-rich environment. The present results could explain reasonably the asymmetric off-stoichiometry of Mg2Ca. The effective point defect concentrations of Mg2Ca as a function of composition and temperature at experimental range are also calculated from a canonical statistical model, and the derived results show a linear relationship between the logarithm of defect concentration and T1. Investigation of geometrical factor reveals that atomic size possesses an obvious influence on the structure of point defect in Mg2Ca. The electronic feature is further studied to reveal underlying mechanism for formation of point defects.Secondly, the density function theory calculations are performed to study the native point defect in MgZn2. The calculated defect formation energy depends largely on the atomic chemical potentials of Mg and Zn, and suggests that defect of Mgzn2 is dominant under strong Mg-rich condition, while VZn2 is favorable under moderate Mg-rich condition. For Zn-rich side Zn anti-site on Mg sublattice is energetic favorable. Lattice vibration effect is further considered, it is found that incorporation vibration contribution into defect formation energy results in obvious temperature effect. The present results indicate that defect configuration is sensitive to the environment conditions, reasonably explain the different experimental observations. The derived point defect concentration undergoes a dramatic change from Zn- to Mg-rich near the stoichiometry. The locally atomic geometry around point defect and electronic structure are also studied, which further reveal the underlying mechanism for structure of point defect.