Anomalous layer expansion in thin niobium films during hydrogen absorption

Hydrogen absorption in Nb layers of Fe/Nb and W/Nb films was explored in situ by small-angle neutron/ x-ray reflectometry, high-angle x-ray diffraction, and EXAFS spectroscopy. In addition to the hydrogen-induced Nb lattice expansion, the experiments reveal an anomalously large thickness expansion of the Nb layers. This expansion is interpreted to be caused by the rearrangement of Nb atoms, and the creation of additional lattice planes in response to the hydrogen-induced lateral lattice strain. This effect is likely the origin of the extraordinarily large hydrogen concentrations found in these films.