On the mechanism of sulfur fast diffusion in 3-D transition metals

The origin of abnormally fast diffusion of sulfur in face-centered cubic (fcc) nickel was investigated within an ab initio band structure calculation approach. A vacancy-mediated model of substitutional impurity diffusion with first-principles calculated parameters was used to determine the diffusion coefficients of S and Al impurities in fcc Ni. The sulfur diffusion coefficient was found to be two orders of magnitude higher than that of aluminum, in good agreement with experimental data. We demonstrate that ultrafast diffusivity of sulfur arises from its chemical interaction with the Ni matrix. The valence electron transfer from Ni3d-Ni3d bonds to sulfur softens the Ni-Ni bonding. That increases the sulfur-vacancy exchange rate, the sulfur-vacancy binding energy and the rotation rate of vacancy around sulfur. The latter is shown to be the key factor that governs S diffusion. We also discuss the impact of Ni-Ni bond softening on the mechanical properties of Ni.