The temperature-dependent diffusion coefficients of interstitial hydrogen, deuterium and tritium were computed using transition state theory. The coefficient of thermal expansion, enthalpy and entropy of activation, and the pre-exponential factor for diffusion were deduced from ab initio total energy and phonon calculations; including the vibrations of all atoms. Numerical results revealed that diffusion between octahedral interstitial sites occurred along an indirect path, via the metastable tetrahedral site, and that both the migration enthalpy and entropy were strongly temperature dependent. The migration enthalpy and entropy were coupled so that the diffusion coefficient was a constant:

D:      D(m2/s) = 2.40 x 10-6exp[-44.09(kJ/mol)/RT]

H:      D(m2/s) = 3.84 x 10-6exp[-45.72(kJ/mol)/RT]

T:      D(m2/s) = 1.77 x 10-6exp[-43.04(kJ/mol)/RT]

The diffusion of deuterium and tritium was estimated to be slower than that of hydrogen only at temperatures above 400K. At lower temperatures, the order was reversed; in excellent agreement with experiment.

Temperature-Dependent Diffusion Coefficients from ab initio Computations: Hydrogen, Deuterium, and Tritium in Nickel. E.Wimmer, W.Wolf, J.Sticht, P.Saxe, C.B.Geller, R.Najafabadi, G.A.Young: Physical Review B, 2008, 77[13], 134305