Vacancy trapping mechanism for hydrogen bubble formation in metal

We reveal the microscopic vacancy trapping mechanism for H bubble formation in W based on first-principles calculations of the energetics of H-vacancy interaction and the kinetics of H segregation. Vacancy provides an isosurface of optimal charge density that induces collective H binding on its internal surface, a prerequisite for the formation of H-2 molecule and nucleation of H bubble inside the vacancy. The critical H density on the vacancy surface before the H-2 formation is found to be 10(19)-10(20) H atoms per m(2). We believe that such mechanism is generally applicable for H bubble formation in metals and metal alloys.