The role of tunnelling in light atom diffusion in metals: A comparison of resonant and non-resonant theoretical approaches

We are concerned here with the question of the applicability of resonant (energy splitting) and nonresonant (transmission coefficient) tunnelling theories to the problem of symmetric double well transfer. We apply a recently developed method of the former type, the Generalized Method of Beats (GMOB), to the calculation of interstitial hydrogen and deuterium diffusion in the metals niobium, vanadium and palladium and compare our results to both experiment and earlier calculations using a nonresonant theory. Both theories can be parametrised so as to fit approximately the experimental temperature dependence of the diffusion coefficients, but the parameters differ markedly. For fixed barrier height, the mean jump distance needed to fit a given rate using the resonant theory is jump distance in the resonant approach is 2.0 Å, which is close to an experimental value; in a nonresonant approach by Weiner a very much smaller distance, 1.166 Å, was required. It is noted, however, that the appropriateness of the underlying model assumptions to the interpretation of hydrogen/deuterium migration dynamics in metal requires further investigation.