Temperature- and pressure-induced structural transitions in rare-earth-deficient R_1-xNi₂ (R = Y, Sm, Gd, Tb) Laves phases

Electrical resistivity, thermal expansion, and temperature-dependent x-ray diffraction measurements on R_1-xNi₂ compounds give mutually consistent evidence for structural phase transitions at 740 K, 550 K, 600 K, and 450 K respectively for R = Y, Sm, Gd, Tb; 0 < x < 0.05. Arguments 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 temperature and at ambient pressure. This superstructure with the space group F43m and a doubled cell parameter is characterized by ordered vacancies on the R sites. It is shown that the observed structural instabilities result in transitions to the cubic Laves phase (space group Fd3m), however with disordered vacancies at high temperatures. High-pressure x-ray powder diffraction experiments show that the phase transition in Y_0.95Ni₂ shifts down to room temperature for a pressure of 27 GPa.