It was noted that, when H was absorbed by Ta, the metal lattice expanded appreciably in proportion to the H content. It was confirmed that X-ray determinations of the lattice parameter could be used to measure H concentrations. This technique was used to monitor changes in H concentration profiles in Ta at 30, 58 and 85C. The results, when extrapolated to zero H concentration, could be fitted to:

D (cm2/s) = 1.8 x 10-6 exp[-3.5(kcal/mol)/RT]

The activation energy was essentially independent of the H concentration.

H.Züchner: Zeitschrift für Physikalische Chemie, 1972, 82[5-6], 240-55