Indications of flat bands driving the <i>δ</i> to <i>α</i> volume collapse of plutonium.

Abstract

On cooling from the melt, plutonium (Pu) undergoes a series of structural transformations accompanied by a ≈ 28% reduction in volume from its <i>δ</i> phase to its <i>α</i> phase at low temperatures. While Pu's partially filled 5<i>f</i>-electron shells are known to be involved, their precise role in the transformations has remained unclear. By using calorimetry measurements on <i>α</i>-Pu and gallium-stabilized <i>δ</i>-Pu combined with resonant ultrasound and X-ray scattering data to account for the anomalously large softening of the lattice with temperature, we show here that the difference in electronic entropy between the <i>α</i> and <i>δ</i> phases dominates over the difference in phonon entropy. Rather than finding an electronic specific heat characteristic of broad <i>f</i>-electron bands in <i>α</i>-Pu, as might be expected to occur within a Kondo collapsed phase in analogy with cerium, we find it to be indicative of flatter subbands. An important role played by Pu's 5<i>f</i> electrons in the formation of its larger unit cell <i>α</i> phase comprising inequivalent lattice sites and varying bond lengths is therefore suggested.