Phase stabilization by electronic entropy in plutonium
Abstract Plutonium metal undergoes an anomalously large 25% collapse in volume from its largest volume δ phase ( δ -Pu) to its low temperature α phase, yet the underlying thermodynamic mechanism has largely remained a mystery. Here we use magnetostriction measurements to isolate a previously hidden yet substantial electronic contribution to the entropy of δ -Pu, which we show to be crucial for the stabilization of this phase. The entropy originates from two competing instabilities of the 5 f -electron shell, which we show to drive the volume of Pu in opposing directions, depending on the temperature and volume. Using calorimetry measurements, we establish a robust thermodynamic connection between the two excitation energies, the atomic volume, and the previously reported excess entropy of δ -Pu at elevated temperatures.