First-principles total-energy calculations were made of the structural stability of Ni2Y1-x. This was often considered to exhibit the cubic Laves phase structure, but X-ray diffraction experiments had shown that it had a C15 superstructure, with ordered Y vacancies; giving a stoichiometry of about Ni2Y0.95. Total-energy calculations for the superstructure and for the ideal C15 structure, as well as for the neighboring phases (NiY, Ni3Y) in the Y-Ni phase diagram, confirmed that the formation of a superstructure with Y vacancies was favoured over the formation of the pure C15 compound, Ni2Y. The calculated relaxations of the atoms around the vacancies were also in good agreement with the experimental results; thus demonstrating that the relaxation of strains in the Y sub-lattice was the driving mechanism for the formation of vacancies.

Ab initio Studies of the Formation of a Y1-xNi2 Superstructure with Ordered Y Vacancies. A.Lindbaum, J.Hafner, E.Gratz: Journal of Physics - Condensed Matter, 1999, 11[5], 1177-87