Anisotropic diffraction peak broadening and dislocation substructure in hydrogen-cycled LaNi₅ and substitutional derivatives

Anisotropic peak broadening in hydrogen-cycled hexagonal LaNi₅ and substitutional derivatives has been studied by means of synchrotron powder diffraction. The data have been analysed by a local lattice parameter variation method implemented in a Rietveld code and by an individual profile fitting using a dislocation peak broadening model. Two main dislocation systems, both with Burgers vector 1/3<110>, are activated by misfit of the lattice parameters between the intermetallic compounds and their hydrides. Two types of diffraction peak broadening effect were observed as a function of the substitution in LaNi₅: (i) a decrease or disappearance of the broadening related to the decrease of the total dislocation density and (ii) a change in the anisotropy of the broadening related to the change of the nature of the dislocation system involved. The latter effect was attributed to a change in shape of the hydride precipitates.