Quantum transition state theory and the diffusion of hydrogen in metals

Abstract

The diffusion of hydrogen in metals is dominated at low temperatures by transitions between vibrational quantum ground states, but at high temperatures quantum effects are expected to be small. These ideas are illustrated by the behaviour of simple barrier-crossing models. The recently developed quantum transition state theory is outlined. Its ability to span both the quantum and the classical regimes is noted, and it is shown how simulation techniques allow it to be used to calculate diffusion coefficients. New simulation results for the diffusion coefficient of hydrogen and deuterium in niobium indicate that the observed change in activation energy at T ≈ 250 K signals a cross-over between quantum and classical regimes. The close relation between this and previous interpretations is stressed.