Indications of flat bands driving the δ to α volume collapse of plutonium

The structural transformations of plutonium (Pu) upon cooling from the melt, specifically from the δ to the α phase, have long puzzled researchers. In this study, we employed calorimetry, resonant ultrasound, and X-ray scattering to elucidate the underlying mechanisms. Our findings reveal that the dominant factor driving these transformations is the difference in electronic entropy between the α and δ phases, rather than phonon entropy. Surprisingly, unlike expectations based on analogous systems, α-Pu exhibits a specific heat characteristic indicative of flatter subbands instead of broad f-electron bands. These results suggest a crucial role for Pu's 5f electrons in the formation of the α phase, with its distinct unit cell and varying bond lengths. This understanding enhances our knowledge of Pu's complex behavior and contributes to the broader understanding of f-electron systems.