TEMPERATURE-INDUCED AND PRESSURE-INDUCED STRUCTURAL TRANSITIONS IN RARE-EARTH-DEFICIENT R(1-X)NI(2) (R=Y,SM,GD,TB) LAVES PHASES

Electrical resistivity, thermal expansion, and temperature-dependent x -ray diffraction measurements on R(1-x)Ni(2) compounds give mutually c onsistent evidence for structural phase transitions at 740 K, 550 K, 6 00 K, and 450 K respectively for R = Y, Sm, Gd, Tb; 0 < x < 0.05. Argu ments are given as to why most of the rare-earth-nickel compounds with the 1:2 ratio do not crystallize in the simple cubic Laves phase (C15 type) but show a superstructure of the cubic Laves phase at room temp erature and at ambient pressure. This superstructure with the space gr oup <F(4)over bar 3m> and a doubled cell parameter is characterized by ordered vacancies on the R sites. It is shown that the observed struc tural instabilities result in transitions to the cubic Laves phase(spa ce group <Fd(3)over bar m>), however with disordered vacancies at high temperatures. High-pressure x-ray powder diffraction experiments show that the phase transition in Y0.95Ni2 shifts down to room temperature for a pressure of 27 GPa.