Transmission electron microscopy study of unhydrided, dehydrided and annealed LaNi₅

Abstract

The influence of hydrogen absorption on the microstructure of LaNi₅ powders has been investigated by transmission electron microscopy. At the surface of the unhydrided and dehydrided LaNi₅ grains a reaction layer is observed. By means of selected area electron diffraction this layer is found to consist of metallic nickel and a second constituent, most probably La₂O₃. This observation suggests that this surface layer is not induced by the hydrogen absorption reaction. Within dehydrided LaNi₅ grains planar-like defects, which are formed during previous hydrogen absorption, were observed. The habit plane, strain field and average spacing of these planar-like defects have been determined. From the correspondence between the above characteristics and the reduced coherence length perpendicular to the c-axis, as derived from anisotropic peak broadening in X-ray diffraction analyses, it was concluded that a correlation exists between the presence of planar-like defects and peak broadening. This correlation is further sustained by the fact that they both disappear upon annealing at 600°C.