Formation and structure of highly over-stoichiometric LaNi5+x (x similar to 1) alloys obtained by manifold non-equilibrium methods

Highly over-stoichiometric LaNi5+x (x similar to1) alloys have been obtaine d by distinct non-equilibrium preparation routes: ageing of amorphous La-Ni films, melt spinning and mechanical alloying. These alloys are metastable and exhibit a TbCu7-type structure. Refinement of X-ray diffraction data sh ow that overstoichiometry is achieved by a random substitution of some La a toms by c-axis oriented Ni dumbbells followed by a relaxation of basal Ni a toms towards the dumbbell position. Annealing of LaNi5+x (x similar to1) al loys at moderated temperatures (similar to 800 K) produces precipitation of the secondary Ni phase as dictated by the La-Ni equilibrium phase diagram. We propose that metastable La-Ni alloys with TbCu7-type structure are form ed when the atomic mobility in the system is low enough to prevent the diff usion of Ni atoms towards Ni-precipitation centers. The possibility of prep aring these compounds by mechanical alloying enables their production on a large scale for possible industrial applications. (C) 2001 Elsevier Science B.V. All rights reserved.