Self-diffusion mechanisms based defect complexes in MoSi₂

MoSi₂is widely concerned due to excellent electrical conductivity, oxidation resistance as a typical transition metal silicide. The high-temperature diffusion behavior is one of the important factors for the degradation of MoSi₂coatings. However, the diffusion mechanism in MoSi₂is still unclear. Prior theoretical work mostly focused on defect formation energy, but these are not consistent with the self-diffusion experiments because the migration behaviors were not considered. Therefore, the purpose of this work was to investigate the microscopic diffusion mechanisms of Mo and Si atoms in MoSi₂using density functional theory and the CI-NEB method. We confirmed that the temperature-dependent vibrational contribution has a significant impact on the defect formation free energy. The isolated point defects in MoSi₂will tend to aggregate to form defect complexes, which participate in the atomic diffusion as mediators. The defect migration behaviors of atoms for vacancy mediated, vacancy complex mediated, and antisite assisted jumps were obtained based on electronic structures analysis. The results show that Si diffusion is mediated by intrasublattice jumps of the nearest neighbor Si vacancies. Moreover, the destroyed covalent Mo-Si bonds by Si vacancies and the non-directional weak metal bonds formed by the Mo antisites and Mo atoms could improve the mobility of the Mo atom which results in the low migration barrier. The agreement between our calculations and the reported experimental results indicates that the dominant diffusion mechanism for Mo atoms is mediated by vacancy complex mediated jumps and antisite assisted jumps. It is concluded that the Si vacancy-based defect complexes are likely the diffusion mediators for Mo atom self-diffusion in MoSi₂. This work provides a deeper insight into the connection between the atomic mechanism and the macroscopic behavior for the diffusion in the MoSi₂, and establishes the basis for further optimizing high-temperature coating materials.