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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 (P6/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 3g 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 of 10 to15 K. The TbNi3.5Al1.5 hydride shows a reinforcement of the magnetic couplings with Tc = 105 K. This leads to assume that nickel has got a magnetic polarisation but reverse to terbium, owing to the J(4f) = IL + SI with M(d) coupling.

RNi5 compounds and their hydrides; neutron diffraction; magnetic properties

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