Direct Monitoring of Vacancy Thermal Equilibration.

Vacancy thermal equilibration (VTE) is ubiquitous, and it plays a crucial role in many complex physical and chemical processes. However, because it has not yet been possible to directly monitor the VTE process, the mechanism of VTE remains unclear. In this work, we realize such direct monitoring for the first time via molecular dynamics (MD) simulations of aluminum slabs with free surfaces and an innovative method for identifying vacancies. By developing a diffusion model to fit the MD data, we verify that the VTE process obeys Fick's second law. The vacancy concentration to which the system eventually converges is found to be the thermodynamic equilibrium concentration, independent of the structure and orientation of the surface. The key diffusion coefficient is found to be an equivalent value. It decreases as the degree of vacancy aggregation increases. This first monitoring of VTE will have profound implications for many vacancy-mediated fields.