Pressure modulated charge transfer and phonon interactions drive phase transitions in uranium-aluminum laves phases

Lanthanide AB(2) intermetallic compounds known as Laves phases have itinerant 3d and localized 4f electrons, which lead to interesting physical properties such as magnetic anisotropy and high Curie temperatures. Actinide Laves phases can display physical properties that are similarly intriguing. However, at reduced A-A spacing the C14 and C15 polytypes may exhibit larger wavefunction overlap for their 5f electron states and distinct characteristics for phases with more delocalized chemical bonding. The C36 polytype, on the other hand, is extraordinarily rare (<5% of known Laves phases). UAl2 is the only known actinide Laves phase to show a pressure-controllable C15 -> C36 transition. Here, we apply first principles calculations to determine the origin of the C15 -> C36 phase transition and reveal the differences between the corresponding properties of each phase. Pressure increases lead to bond compression-induced electron transfer from Al to U, which drives dynamic instability in the C15 phonon modes because of the uniform U-U bonding environment. The opposite phenomena is observed in C36: varied U-U bonding environments are vibronically more stable after charge transfer. We find that the interplay between charge transfer, chemical bonding, and phononic stability are central to predicting phase transitions and corresponding changes in physical properties for both C15 and C36 UAl2.