Investigating the Role of Phonons in the Phase Stability of Uranium based Laves phases

Laves phases are intermetallic alloys which form in one of three specific structural arrangements, denoted C14 (P63/mmc), C15 (Fd-3mS), and C36 (P63/mmc), and comprise interconnected tetrahedra that facilitate high electrical and thermal conductivity. The factors governing the formation of a Laves phase into one structure over another remains an open question. In particular, the role of phonons, which may provide information about phase transition mechanisms, transition temperatures, and vibrational contributions to entropy, has been widely overlooked.

We investigate the influence of phonons in the UCo_xNi_2-x uranium metal alloy system. This system is an ideal test case because it presents all three Laves phase structures depending on the Co:Ni ratio. Analysis of phonon density of states, band structure, and eigenvector decomposition obtained via density functional theory (DFT), will be presented for all three structure types. Specific attention will be given to the existence of imaginary modes and a unique flat phonon band in the C15 structure.