STRUCTURAL INSTABILITY IN R1-XNI2 COMPOUNDS AND THEIR HYDRIDES (R=Y, RARE-EARTH)

RNi2 Laves phases crystallize with ordered R vacancies in a cubic structure derived from the C15 type cubic structure (Fd3m space group) by doubling the cell parameter, the symmetry is lowered and the new space group is F-43 m. The vacancies allow structural relaxation and as the radius of the R atoms (R = Pr to Lu) decreases the amount of ordered vacancies on the 4a site decreases: for LuNi2 the structure is almost C15. R1-xNi2 compounds form hydrides which are crystalline or amorphous depending on the R atom, the hydrogen content and the absorption conditions. Structural studies for Nd0.95Ni2, Tb0.95Ni2 and Er0.98Ni2 hydrides show that when the radius of the rare earth decreases the formation of an amorphous hydride occurs at a larger H content. A relation between the structural instability of the intermetallic compound and the formation of an amorphous hydride is therefore proposed.