Theoretical study of the ferromagnetism in UNi2

Investigating the role of phonons in the phase stability of uranium-based Laves phases

Laves phase alloys possess unique thermal and electrical conduction properties, yet the factors governing phase stability in these systems remain an open question. The influence of phonons in particular has been broadly overlooked. Here, we investigate the UCo_2x Ni_2(1-x) chemical space using density functional theory, which offers a unique opportunity to explore the factors influencing Laves phase stability as all three primary Laves phases (C14, C15, C36) can be stabilized by changing the ratio of Co to Ni. Calculations of the thermodynamic and dynamical stability of pure UCo₂ and UNi₂ in each of three primary Laves phases confirm the stability of experimentally known Laves phases for UNi₂ and UCo₂. A decrease in bonding strength is identified in UNi₂ compared to UCo₂, aligned with redshifts observed in the UNi₂ phonon density of states and a decoupling of the U and Ni vibrational modes. Phonon calculations of C14 UCo₂ reveal dynamical instabilities. Efforts to remove the unstable mode at the Γ point in UCo₂ via atomic displacements break the symmetry of the C14 phase, revealing a lower energy P2/c structure. Vibrational contributions to the free energy were calculated and did not change the thermodynamically stable Laves phase below 1000 K. The temperature-dependent free energies of single phase UCo₂ and UNi₂ were used to interpolate the relative stability of ternary UCo_2x Ni_2(1-x) in each of the three Laves phases at varying temperatures and stoichiometries. The ternary C36 phase is only predicted to be thermodynamically stable over a narrow stoichiometric range below 600 K.