The serial sectioning technique was used to study the isotope effect for the diffusion of Na in crystals at 589 to 796C (table 130). The isotope effect was found to decrease as the temperature increased (table 131); indicating a vacancy-pair contribution to the self-diffusion of Na in NaCl. This contribution attained 30 to 45% near to the melting point. The Temperature Dependence of the Isotope Effect for the Diffusion of Na+ in NaCl. S.J.Rothman, N.L.Peterson, A.L.Laskar, L.C.Robinson: Journal of Physics and Chemistry of Solids, 1972, 33[5], 1061-9