Some comments on the mass effect in diffusion

Abstract

An alternative derivation is given of Mullen's (1961) relation between the ratio of the diffusion jump frequencies of two isotopes wα/wβ and the ratio of their masses mα/mβ. A connection is suggested between the kinetic energy factor δK and relaxation around defects. It is pointed out that when δK≠1 Schoen's equation (1959) for the isotope effect cannot be exact and the nature of the corrections necessary is discussed. Finally, quantum corrections to the classical rate equations for atomic jump frequencies are discussed and shown to be important in considering mass effects for diffusion in metals of light interstitial solutes such as H₂ and D₂.