Investigation of nanocrystalline Gd films loaded with hydrogen

•Microstructure development of hydrogenated nanocrystalline Gd films was studied.•The virgin film contains already traces of the β-phase (GdH2). With increasing hydrogen concentration Gd gradually transforms into the β-phase.•The region of α+β phase coexistence is narrower compared to bulk Gd.•The γ-phase (GdH3) is not formed.•Prolonged hydrogen loading leads to amorphization of the film structure. The present work reports on microstructure studies of hydrogen-loaded nanocrystalline Gd films prepared by cold cathode beam sputtering on sapphire (112¯0) substrates. The Gd films were electrochemically step-by-step charged with hydrogen and the structural development with increasing concentration of absorbed hydrogen was studied by transmission electron microscopy and in-situ X-ray diffraction using synchrotron radiation. The relaxation of hydrogen-induced stresses was examined by acoustic emission measurements. In the low concentration range absorbed hydrogen occupies preferentially vacancy-like defects at GBs typical for nanocrystalline films. With increasing hydrogen concentration hydrogen starts to occupy interstitial sites. At the solid solution limit the grains gradually transform into the β-phase (GdH2). Finally at high hydrogen concentrations xH>2.0 H/Gd, the film structure becomes almost completely amorphous. Contrary to bulk Gd specimens, the formation of the γ-phase (GdH3) was not observed in this work.