The temperature dependence of the positron annihilation peak coincidence rate was measured in iron from 20 to 1320C. The peak rate did not change across the ferro-para magnetic transition at approximately 770C but decreased by 1.5% across the alpha - gamma phase transition at 910C. The trapping model analysis of the data yielded values of the vacancy formation energies of 1.4eV in alpha -Fe and 1.7eV in gamma -Fe.

Vacancy Formation Energy in Iron by Positron Annihilation. S.M.Kim, W.J.L.Buyers: Journal of Physics F, 1978, 8[5], L103-8