The Nb layers of a [100nmNb/2.6nmFe]5 multi-layer were repeatedly charged with H from the gas phase. The equilibrium H concentrations in the Nb layers, and the H-induced layer-thickness expansion perpendicular to the film plane, were determined as a function of pressure by means of in situ neutron reflectivity. X-ray diffraction experiments which were performed on the same sample yielded the corresponding expansion of the out-of-plane Nb(110) interplanar spacing and the time-dependence of the charging and de-charging processes. It was found that the relative expansion of the Nb layers was considerably larger than the relative increase in the Nb(110) interplanar spacing. This showed that, during H incorporation, a large additional volume - presumably in the form of voids in the vicinity of grain boundaries - was created. These lattice imperfections could effectively trap H atoms. The maximum H concentration, at a H2 partial pressure of 0.9bar and a temperature of 18C, was 95[H]/[Nb]at%.

Hydrogen Concentration and its Relation to Interplanar Spacing and Layer Thickness of 1000Å Nb(110) Films during in situ Hydrogen Charging Experiments. C.Rehm, H.Fritzsche, H.Maletta, F.Klose: Physical Review B, 1999, 59[4], 3142-52