Structure of LaNi2.286 and the La-Ni system from LaNi1.75 to LaNi2.50

Article abstract of DOI:10.1016/0022-5088(88)90357-8

Seven alloys near the LaNi₂ composition were studied by metallographic and X-ray techniques. Alloys that were cooled slowly were found to consist of two phases: from x=1.75 to 2.25 and from x=2.40 to 2.50, where x is the Ni:La ratio. A single-phase alloy was found near x=2.30. A complete single-crystal structure analysis showed that the true composition of this tetragonal phase (a=7.355 Angstrom and c=14.51 Angstrom) is LaNi_2.286. The space group is I42m and the unit cell contains 32 nickel atoms and 14 lanthanum atoms. This tetragonal structure is related to the cubic C15 Laves phase such that a_t approximately equals a_c and c_t approximately equals 2a_c and two lanthanum atoms are removed from the tetragonal structure. The formation of this phase is thought to result from the large disparity between the lanthanum and nickel radii (r_La/r_Ni=1.506) relative to that expected for the ideal constituent atoms of the AB₂ cubic Laves phase (r_A/r_B=1.225), which prevents the formation of the ideal C15 Laves phase.