It was pointed out that, when an intrinsic defect (vacancy, self-interstitial) mediated self-diffusion, the activation enthalpy was equal to the sum of the enthalpies for defect formation and defect migration. However, if the migration enthalpy was small and the migration entropy was large then, at some critical temperature, the saddle point became the stable configuration and a totally different path could become dominant. Therefore, it was possible that paths of higher activation enthalpy could predominate at high temperatures and lead to a curvature of the Arrhenius plot. These effects were very likely to occur in crystalline Si, and might account for the large entropy observed in high-temperature self-diffusion.

S.T.Pantelides: Physical Review B, 1987, 36[6], 3462-4