A serial sectioning technique was used to study the self-diffusion of 22Na in high-purity crystals in the extrinsic range. The data for temperatures below 550C could be described by:

D (cm2/s) = 3.55 x 10-6 exp[-0.839(eV)/kT]

It was concluded that, even in pure crystals, there were still enough cation vacancies (some 10-6mol%) to account for single cation vacancy migration.

R.Reisfeld, W.J.Fredericks: Israel Journal of Chemistry, 1971, 8[6], 959-62