Pressure-induced order-disorder transitions in R Ni₂ compounds

Most of the RNi₂ compounds (R= rare-earth element) do not crystallize in the cubic Laves phase structure (C15), but in a superstructure of C15 with ordered vacancies at the R sites. In the present work high pressure x-ray diffraction experiments on selected RNi₂ compounds (R=Tb,Sm,Gd, and Y) are presented, showing that pressures of about 8-15 GPa lead to a disordering of the vacancies in all of them. Furthermore the studies show that the pressure-induced order-disorder transition is closely related to the earlier observed disordering of the vacancies at high temperatures between 430 (TbNi₂) and 740 K (YNi₂), and that there is a pronounced variation of the transition pressures and temperatures among the investigated compounds. The driving mechanism behind these transitions is discussed.