Neutron diffraction investigations and magnetic properties of RNi3.5Al1.5 compounds and their hydrides (deuterides)

When annealed, the RNi3.5Al1.5 (R = Y, Tb, Dy, and Ho) compounds were found to crystallise with the hexagonal CaCu5-type structure (P 6/mmm). Hydrogenation under 1.5 MPa gas pressure, shows that the maximum hydrogen capacity varies with the rare earth atom from 3.8, 2.8, 3.9 to 2.42 for Tb, Dy, Ho and Y, respectively. Analysis of neutron diffraction patterns shows that Al atoms occupy only the 3 g sites. The lattice parameter expansion can be correlated both to the size of the R atom and to the amount of aluminium. Magnetisation measurements show strong ferromagnetic contribution supported by magnetic R elements, e.g. R = terbium. But the exchange couplings remain rather weak, corresponding to a Curie temperature Tc comprised from 10 to 15 K. The TbNi3.5Al1.5 hydride reveals a reinforcement of the magnetic couplings with Tc=105  K. This leads to assume that nickel is magnetically polarised but reverse to terbium, owing to the J(4f)=|L+S| with M(d) coupling.