First-principles study of vacancy formation and migration energies in tantalum

The vacancy formation and migration enthalpies H-f and H-m in tantalum are calculated by means of ab initio pseudopotential techniques. The results obtained after relaxation with supercells containing 54 atomic sites (H-f = 3.0 eV and H-m = 0.8 eV) are in excellent agreement with experiments (H-f = 3.1 eV and H-m = 0.7 eV). The electronic entropy has a moderate influence on their temperature dependence. The effects of structural relaxation, which reduce H-f by 14%, are significantly larger than for group-VI elements and are typical of a normal behavior for the bcc structure. The experimental tracer self-diffusion coefficient is well reproduced within the rate theory using an estimate of the vibrational formation entropy and the attempt frequency. [S0163-1829(99)12133-7].