The volume diffusivity of Hf in monocrystalline W samples was studied, at 2023 to 2573K, by means of secondary-ion mass spectrometry. The results were described by:

D(cm2/s) = 2.19 x 100 exp[-132.8(kcal/mol)/RT]

It was found that the activation enthalpy for the volume diffusion of such a 5d element depended parabolically upon the difference in valence of the impurity and host atom.

S.M.Klotsman, S.V.Osetrov, A.N.Timofeev: Physics of Metals and Metallography, 1996, 81[6], 668-74

The best linear fits to the solute diffusion data ([5] to [25]) yield:

H (sd): Ln[Do] = 1.49E – 21 (R2 = 0.87); W(sd): Ln[Do] = 0.33E – 16.1 (R2 = 0.63)