QUANTUM DIFFUSION OF HYDROGEN IN THE BCC TRANSITION-METALS

The auxiliary function governing the temperature dependence of the sma ll-polaron diffusion coefficient D(T) is calculated with improved accu racy for impurities occupying tetrahedral and octahedral interstitial sites in tantalum, niobium, and vanadium. This function depends only o n the host crystal and determines the shape of the curve D versus T. T he nature of the diffusing species enters as just a scale factor and d isplacement of the Arrhenius plot of D(T). The calculation considers t he real vibrational modes of the host crystals and employ state-of-the -art integration techniques over the Brillouin zone to give highly rel iable and accurate results. Tables of use in the interpretation of fur ther experiments are given. As an example of application, the diffusio n data of hydrogen in tantalum, available for temperatures in the rang e 15 K < T < 500 K, are analyzed on the basis of the quantum theory of diffusion and a two-states model.