Ion-driven permeation was used to determine the diffusivity, rather than gaseous or electrochemical charging. The steady-state permeation rate and the diffusion coefficient tended to decrease with increasing fluence, because of changes in the surface or sub-surface regions. It was found that the results, at temperatures of between 500 and 1100K, could be described by:

D (m2/s) = 1.4 x 10-6 exp[-35.7(kJ/mol)/RT]

Y.Furuyama, T.Tanabe, S.Imoto: Journal of the Japan Institute of Metals, 1986, 50[8], 688-94