Ab initio calculations on the formation of La_1-xNi₂ compounds

First principle calculations have been performed to compare the stability of the stoechiometric LaNi₂ compound, crystallizing in the cubic C15 structure, with that of the C15 superstructure compound La₁₅Ni₃₂ and the tetragonal La₇Ni₁₆ which both contain La vacancies. These results show that LaNi₂ is not stable against decomposition in the two neighbouring phases La₂Ni₃ and LaNi₃ of the La-Ni phase diagram, which means that LaNi₂ cannot be formed. The formation of La vacancies allows the relaxation of the stresses caused by too short La-La distances and lowers the enthalpy of formation. The La₇Ni₁₆ phase is more stable than the cubic La₁₅Ni₃₂, but the enthalpy of formation of La₁₅Ni₃₂ lies only slightly above the critical value for stability. This small difference may explain that, with very pure lanthanum, the formation of a cubic La₁₅Ni₃₂ phase is not observed, but can be stabilized by the presence of impurities present in commercial La.