Hydrogen absorption in epitaxial W/Nb(0 0 1) and polycrystalline Fe/Nb(1 1 0) multilayers studied in situ by X-ray/neutron scattering techniques and X-ray absorption spectroscopy

Hydrogen can be absorbed in large quantities by 100 angstroms thin Nb layers embedded in epitaxial W/Nb and polycrystalline Fe/Nb multilayers. The solubility and the hydrogen-induced structural changes of the host lattice are explored in situ by small-angle neutron/X-ray reflectometry and high-angle diffraction. These measurements reveal for both systems that the relative out-of-plane expansion of the Nb layers is considerably larger than the relative increase of the Nb interplanar spacing indicating two distinctly different mechanisms of hydrogen absorption. In Fe/Nb multilayers, hydrogen expands the Nb interplanar spacing in a continuous way as function of the external pressure. In contrast, the Nb lattice expansion is discontinuous in epitaxial W/Nb multilayers: A jump in the Nb(0 0 2) Bragg reflection position occurs at a critical hydrogen pressure of 1 mbar. In situ EXAFS spectroscopy also exhibits an irreversible expansion of the Nb lattice in the film plane for p_H(2)>1 mbar. This can be regarded as a structural phase transition from an exclusively out-of-plane to a three-dimensionally expanded state at low and high hydrogen pressures, respectively.