Pressure-induced order-disorder transitions in (formula presented) compounds

Most of the (formula presented) compounds (formula presented)=rare-earth element) do not crystallize in the cubic Laves phase structure (formula presented) but in a superstructure of (formula presented) with ordered vacancies at the R sites. In the present work high pressure x-ray diffraction experiments on selected (formula presented) compounds (formula presented) and (formula presented) 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 (formula presented) and 740 K (formula presented) 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.