Theory of the self-diffusion coefficient in cubic metals

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Abstract
The kinetics of diffusion in cubic metals is treated in terms of statistical mechanics from the point of view of absolute rate theory. An approximate description of the activated state leads to the conclusion that in the two degrees of freedom orthogonal to the jump direction only the lower vibrational energy levels are occupied. This circumstance gives rise to a negative contribution to the experimental entropy of activation for diffusion in solids which is negligible for the vacancy mechanism but very appreciable for the ring mechanism. It is suggested that a low D 0, perhaps of the order of 10−4 cm2/sec, is an experimental criterion for the 4-atom ring mechanism and that the self-diffusion in pure chromium and uranium (b.c.c.) may occur by this mechanism. Further, a low Arrhenius frequency factor is predicted for all thermally activated processes in solids which require simultaneous jump of n atoms, e.g. in certain movements of dislocation loops.