Structural study of the pseudobinary Y(Ni,Cu)2 system

The pseudobinary Y(Ni1−xCux)2 compounds have been investigated by X-ray and neutron diffraction, density measurements as well as with electron probe microanalysis. The goal was to understand the mechanism behind the structural change from the cubic structure of Y0.95Ni2 (a cubic superstructure of the cubic Laves phase with ordered vacancies on the Y site) into the orthorhombic CeCu2 type structure of YCu2. 20% of Cu can be substituted for Ni in Y0.95Ni2 accompanied by an increase of the number of Y vacancies in the cubic superstructure. On the Cu-rich side a solid solution was found from YCu2 to Y(Ni0.5Cu0.5)2. The substitution for Ni in the orthorhombic structure mainly causes a decrease of the lattice parameter a, giving rise to a 3.5% contraction of the unit cell volume. These two solid solutions are separated by a two-phase region. The thermal expansion among the cubic compounds hardly changes with the Cu concentration, whereas it increases in the orthorhombic range towards YCu2.