Hydrogen concentration and its relation to interplanar spacing and layer thickness of 1000-angstrom Nb(110) films during in situ hydrogen charging experiments

The Nb layers of a [1000-Angstrom Nb/26-Angstrom Fe]x5 multilayer were repe atedly charged with hydrogen from the gas phase. The equilibrium hydrogen c oncentration in the Nh layers and the hydrogen-induced layer thickness expa nsion perpendicular to the film plane were determined as a function of pres sure by means of in situ neutron reflectivity measurements. In situ x-ray-d iffraction experiments performed on the same sample yielded the correspondi ng expansion of the out-of-plane Nb(110) interplanar spacing and the time-d ependence of the charging and decharging process. It was found that the rel ative expansion of the Nh layers is considerably larger than the relative i ncrease of the Nb(110) interplanar spacing. This shows chat 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 imp erfections may effectively trap hydrogen atoms. The maximum hydrogen concen tration at p(H2) = 900 mbar and T = 185 degrees C is found to be 95 [H]/[Nb ]at. %. [S0163-1829(99)15003-3].