An anomalous energy pathway with energetically equivalent double saddle points for vacancy mediated self-diffusion within an hexagonal close-packed Ti basal plane was unveiled by density functional theory. Examination of migration pathway and phonon force constants suggested that the migrating atom tries to follow the bcc-hcp phase transition via the Burgers shear deformation. It was proposed that the formed energy local minimum with a body-centered cubic-like atomic environment between the two saddle points originates from the existence of high-temperature body-centered cubic phase and was a feature of group-IV hexagonal close-packed metals with a body-centered cubic to hexagonal close-packed phase transition. Computed diffusion coefficients were in good accord with experiments for hexagonal close-packed Ti.

Anomalous Energy Pathway of Vacancy Migration and Self-Diffusion in HCP Ti. S.L.Shang, L.G.Hector, Y.Wang, Z.K.Liu: Physical Review B, 2011, 83[22], 224104