Hydrogen concentration and its relation to interplanar spacing and layer thickness of 1000-Å Nb(110) films duringin situhydrogen charging experiments

The Nb layers of a [1000-Nb/26-Fe]×5 multilayer were repeatedly charged with hydrogen from the gas phase. The equilibrium hydrogen concentration in the Nb layers and the hydrogen-induced layer thickness expansion perpendicular to the film plane were determined as a function of pressure by means of in situ neutron reflectivity measurements. In situ x-ray-diffraction experiments 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 decharging process. It was found that the relative expansion of the Nb layers is considerably larger than the relative increase of the Nb(110) interplanar spacing. This shows that during hydrogen incorporation a large amount of additional volume, presumably in the form of voids in the vicinity of grain boundaries, is created. These lattice imperfections may effectively trap hydrogen atoms. The maximum hydrogen concentration at pH2=900mbar and T=185^∘C is found to be 95 [H]/[Nb] at. %.