Homogeneity range and order-disorder transitions in R1-xNi2 Laves phase compounds

The homogeneity range of R1-xNi, 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 froth 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 R1-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. (C) 2005 Published by Elsevier Ltd.