Homogeneity range and order-disorder transitions in R_1-xNi ₂ Laves phase compounds

The homogeneity range of R_1-xNi₂ Laves phases (R=Ce, Gd, Tb) has been studied by X-ray diffraction, microprobe analysis and density measurements. In these compounds, the number of R vacancies varies as a function of the atomic number of the rare earth metal and the nominal composition. For R=Tb the number of vacancies varies by 0≤x≤0.5, with a change of the structure from a pure C15 structure to a 2a superstructure accompanied by a cell volume decrease. The order-disorder transition of the rare-earth vacancies has been studied under applied pressure for La₇Ni₁₆ and as a function of temperature for all the R_1-xNi₂ compounds. The evolution of the transition pressure and temperature, which reflects the binding energy of the vacancies in the lattice, depends not only on the radius of the R atoms but also on their masses.