Neutron diffraction investigations and magnetic properties of RNi 3.5Al 1.5 compounds and their hydrides (deuterides)

Abstract

When annealed, the RNi 3.5Al 1.5 (R = Y, Tb, Dy, and Ho) compounds were found to crystallise with the hexagonal CaCu 5-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 T c comprised from 10 to 15 K. The TbNi 3.5Al 1.5 hydride reveals a reinforcement of the magnetic couplings with T c = 105 K. This leads to assume that nickel is magnetically polarised but reverse to terbium, owing to the J ( 4 f ) = | L + S | with M(d) coupling.