Local and long-range hydrogen diffusion in Nb(OH)0.011

The quantum diffusion of hydrogen trapped by oxygen interstitials in niobium is well studied in the temperature range below 150 K. The studies demonstrate that the hydrogen is trapped by the oxygen impurity atoms and that the trapped hydrogen performs a rapid local diffusion process between two nearest-neighbour tetrahedral interstitial sites. With temperatures rising above 150 K, the probability that the hydrogen leaves the trap sites increases. The hydrogen dissociates from its original oxygen trapping atom and carries out a long-range diffusional motion before it is trapped again by an oxygen impurity. We studied this situation by neutron scattering in the temperature range from 100 to 300 K. In our experiment we found a continuous transition from local diffusion at low temperatures to a predominantly long-range diffusion at higher temperatures. This behaviour can successfully be described by a two-state model: one state describes the local diffusion of the hydrogen in the trapped state. The other state describes the long-range diffusion process. Each of these states has a characteristic lifetime, which is also a measure for the probability of the hydrogen to be in this state.