INVESTIGATION OF THE CRYSTALLOGRAPHIC STRUCTURES OF LANI4COD4.4 AND LANI3.55MN0.4AL0.3CO0.75DX (X=2.0 AND 4.6 D F.U.) BY NEUTRON POWDER DIFFRACTION/

The structural properties of the deuterides LaNi3.55Mn0.4Al0.3Co0.75Dx with x=2 and 4.6 D/f.u. and LaNi4CoD4.4 have been investigated by mea ns of powder neutron diffraction. The mono-substituted cobalt compound exhibits anisotropic line broadening and crystallises in a different structure than that observed for the fully charged deuteride LaNi4CoD6 .1. Such effects are not observed for the deuterides of the three-subs tituted compound LaNi3.55Mn0.4Al0.3Co0.75 which can be described in th e P6/mmm space group. Analysis of the diffraction patterns at differen t deuterium compositions shows that the plateau pressure extends from 0.6 to 3.4 D/f.u. Beyond this concentration, a large domain of solid s olution exists with a progressive filling of the deuterium sites with increasing deuterium pressure. (C) 1998 Elsevier Science S.A.