Kinetics of hydrogen absorption and desorption in epitaxial thin niobium films

We have investigated the static and kinetic properties upon hydrogen absorption desorption in epitaxial niobium films by in situ and time-resolved resistivity measurements and compare equilibrium values with respective lattice-parameter changes. We find for the excess resistivity a strong deviation from Nordheim's rule, which is usually applied for describing the resistance change as a function of hydrogen concentration. Instead of a maximum for half of the saturation concentration, we observe a shift of the maximum to higher hydrogen concentrations. Furthermore, we find that the relaxation times for reaching an equilibrium concentration when charging or decharging Nb films with hydrogen exhibit three different regimes with characteristically different time scales. For low hydrogen pressures (concentrations) the relaxation time at a temperature of 473 K is on the order of 10(3) s, for an intermediate hydrogen pressure (concentration) the relaxation time increases by more than an order of magnitude, and beyond 100 mbars it drops to less than 10 s.