The equilibrium concentration of point defects quenched in sodium chloride crystals was determined by density measurements. The activation energy was 1.9eV. Comparing the activation energy obtained using these density measurements, with the energy derived from ionic conductivity, it was found that the binding energy of a vacancy pair was 0.89eV.

A Determination of the Formation Energy of Vacancies in Sodium Chloride by Quenching. J.Pelsmaekers, G.Pellegrini, S.Amelinckx: Solid State Communications, 1963, 1[4], 92-5