Defect studies of hydrogen-loaded nanocrystalline Gd films

The present work reports on microstructure investigations of hydrogen-loaded nanocrystalline Gd films by means of slow positron implantation spectroscopy combined with in situ synchrotron radiation X-ray diffraction. It is found that the virgin films contain a high density of vacancy-like open volume defects at grain boundaries which trap positrons. These defects represent trapping sites also for hydrogen. With increasing hydrogen concentration the transformation from the α- into the β-phase (GdH₂) takes place in the film. Accumulation of hydrogen at grain boundaries causes a decrease of positron localization at defects. The transformation into the β-phase is completed at x_H ≈ 1.6 H/Gd. Contrary to bulk Gd specimens, the γ-phase (GdH₃) is not formed in the nanocrystalline Gd films.