Ab initio studies of the formation of a Y_1-xNi₂ superstructure with ordered Y vacancies

Ab initio total-energy calculations have been performed to study the structural stability of Y_1-xNi₂. In the literature (Villars P and Calvert L D 1985 Pearson's Handbook of Crystallographic Data for Intermetallic Phases (Materials Park, OH: American Society for Metals)) YNi₂ is often considered to show the cubic Laves phase structure, but x-ray diffraction experiments of Latroche et al (J. Less-Common Met. 161 L27) showed that YNi₂ crystallizes in a superstructure of C15 with ordered Y vacancies with a stoichiometry of approximately Y_0.95Ni₂. The total-energy calculations for the superstructure and for the ideal C15 structure, as well as for the neighbouring phases in the Y-Ni phase diagram YNi and YNi₃, confirm that the formation of the superstructure with Y vacancies is favoured against the formation of the pure C15 compound YNi₂. The calculated relaxation of the atoms around the vacancies is also in good agreement with the experimental results (Latroche et al), demonstrating that the relaxation of strains in the Y sublattice is the driving mechanism for formation of vacancies. In addition, the electronic properties of the vacancy superstructure have been examined.