Ab initio studies of the formation of a Y1-x Ni-2 superstructure with ordered Y vacancies

Ab initio total-energy calculations have been performed to study the struct ural stability of Y1-xNi2. In the literature (Villars P and Calvert L D 198 5 Pearson's Handbook of Crystallographic Data for Intermetallic Phases (Mat erials 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 Y0.95Ni2 The total-energy calculations for the superstructur e and for the ideal C15 structure, as well as for the neighbouring phases i n the Y-NI phase diagram YNi and YNi3, confirm that the formation of the su perstructure with Y vacancies is favoured against the formation of the pure C15 compound YNi2. The calculated relaxation of the atoms around the vacan cies is also in good agreement with the experimental results (Latroche et a l), 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.