Hydrogen absorption in epitaxial W/Nb(001) and polycrystalline Fe/Nb(110) multilayers studied in situ by X-ray/neutron scattering techniques and X-ray absorption spectroscopy

Abstract

Hydrogen can be absorbed in large quantities by 100 i%thin Nb layers embedded in epitaxial W/Nb and polycrystalline Fe/Nb multilayers. The volubility and the hydrogen-induced structural changes of the flost lattice are explored in-situ by smallangle neutron/X-ray reflectometry and high-!angle diffraction. These measurements " \ reveal for both systems that the relative out-~f-plane expansion of the Nb layers is considerably larger than the relative increase of the Nb interplanar spacing indicating. two distinctly different mechanisms of hy~ogen absowtion.~Fe~multilayers, hydrogen expands the Nb interplanm spacing in a continuous way as function of the external pressure. In contrast, the F@ lattice expansion is discontinuous in epitaxial W/Nb multilayers: A jump in the Nb(O02) 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~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.

Hydrogen Absorption in Metal Thin Films and Heterostructures Investigated in Situ with Neutron and X-ray Scattering

“…These samples were prepared by ion-beam sputtering at room temperature in an ultra-high vacuum chamber with a base pressure <5ˆ10´9 mbar, where the Nb layers grew strongly (110) textured. Epitaxial Nb layers were prepared by growing W/Nb(001) superlattices at elevated temperatures of 200˝C on etched MgO(001) substrates by magnetron sputtering [42,43]. Both Fe/Nb and W/Nb multilayers had a 5 nm thick Pd capping layer to prevent oxidation and to facilitate hydrogen absorption.…”

“…In order to study the correlation between hydrogen absorption and resulting changes of the structural properties of Nb films, Fe/Nb heterostructures were measured with neutron reflectometry (NR) and X-ray diffraction/reflectometry [41][42][43]. The multilayer geometry is advantageous as the scattering signals are strongly amplified.…”

“…In addition, we observe a significant increase in the Bragg peak intensity with increasing hydrogen absorption due to the increase in scattering contrast between Fe and Nb + H layers. (NR) and X-ray diffraction/reflectometry [41][42][43]. The multilayer geometry is advantageous as the scattering signals are strongly amplified.…”