X-ray diffraction peak broadening and lattice strain in LaNi₅-based alloys

Peak broadening in X-ray powder diffraction (XRD) profiles of LaNi₅-based alloys after hydriding and dehydriding processes was investigated in order to clarify the mechanism of formation of lattice strain in hydriding and dehydriding. The Rietveld method was used to evaluate the degree of peak broadening and to determine anisotropic peak broadening axis for LaNi₅ and LaNi_5-xM_α (M: Mn, Fe, Cu, Al; α = 0.25, 0.5) before hydriding, after activation and after 1000 hydriding-dehydriding cycles. All the alloys studied showed anisotropic broadening vectors of the same direction 〈110〉 after activation. The degree of the peak broadening, however, strongly depended on the substitution elements. Hydriding-dehydriding cycles did not influence the direction of the anisotropic peak broadening axis, while both anisotropic and isotropic peak broadening increased with number of cycles. It was found that the lattice strain analyzed from the peak broadening in X-ray diffraction profiles corresponded to dislocations with Burgers vectors 〈hk0〉 observed by transmission electron microscope.