Global mass dependence of the isotope effect in diffusion

Using the short-memory augmented-rate theory (SM-ART) framework of Toller et al. [Phys. Rev. B 32, 2082 (1985)] we have calculated the isotope effects for diffusion in crystals over wide ranges of the mass of the jumping atom relative to that of the host atom. Calculations have been completed for a variety of interatomic potentials including Lennard-Jones, Morse, Finnis-Sinclair, and hard-core forces. The similar global behavior in all cases points to the dominance of closed-shell repulsive forces in determining the isotope effect for diffusion. These results also clarify the expected behavior of the isotope effect for impurity diffusion in which large mass differences are present. An example of the breakdown of the SM-ART approximation is discussed. © 1988 The American Physical Society.