Phase stabilization by electronic entropy in plutonium*

Plutonium metal undergoes an anomalously large 25\% collapse in volume from its largest volume $\delta$ phase ($\delta$-Pu) to its low temperature $\alpha$ 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 $\delta$-Pu, which we show to be crucial for the stabilization of this phase. The entropy originates from two competing instabilities of the $5f$-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 $\delta$-Pu at elevated temperatures.