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Pressure-induced order-disorder transitions in R Ni 2 compounds

2002; American Physical Society; Volume: 65; Issue: 13 Linguagem: Inglês

10.1103/physrevb.65.134114

1095-3795

A. Lindbaum, E. Gratz, S. Heathman,

Magnetic Properties of Alloys

Most of the $R{\mathrm{Ni}}_{2}$ 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 $R{\mathrm{Ni}}_{2}$ compounds $(R=\mathrm{Tb},\mathrm{Sm},\mathrm{Gd},$ 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 $({\mathrm{TbNi}}_{2})$ and 740 K $({\mathrm{YNi}}_{2}),$ 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.