Kinetic theory of the hydrogen diffusion in metals

The diffusion coefficient D for hydrogen in a metal normally obeys the Arrhenius law: D = D0 exp (-Ea/kBT), where Ea is the activation energy, which represents the barrier potential for the most favorable interstitial channel. The pre-exponential factor D0 can be related to the mean straight path l in the crystal by D0 = (1/3) gvl, where v is the average migration speed and g a statistical weight of order one. The mean straight path l for a pure crystal can be very long, since the migrating atoms cannot readily be scattered or stopped by the (phonon) motion of the regular lattice. This physical picture, reasonable but not widely recognized, is supported by experimental evidence and theoretical considerations. In particular, the distinct isotope effects for hydrogen diffusion in f.c.c. and b.c.c. metals are explained in terms of quantum mechanical zero-point energies associated with the crystal channel potentials. A few experimental tests for the theory are suggested. (author)