First principles study of vacancy and tungsten diffusion in fcc cobalt

We have studied the energetics of vacancy formation and diffusion in fcc cobalt using periodic density functional theory, both with and without including the surface intrinsic error corrections. Aggregation of vacancies is found to be energetically favorable. The vacancy formation energies, with (without) corrections, are computed to be 2.34 (1.71) eV for an isolated vacancy and 2.28-1.92 (1.65-1.29) eV per vacancy for two to six coalesced vacancies, respectively. The corrected (uncorrected) diffusion barrier of an isolated vacancy is 1.19 (0.98) eV. We have found that vacancy formation energies are over-predicted for Co, Fe and Ni when surface intrinsic error corrections are applied. We have also studied substitutional tungsten diffusion in Co. We have identified two sequential vacancy-mediated W diffusion mechanisms in Co. Corrected (uncorrected) energy barriers for the steps in these mechanisms lie in the range 1.09-1.44 (0.88-1.23) eV.