Influence of point defects and impurities on the dynamical stability of δ-plutonium

Phase stabilization by electronic entropy in plutonium

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 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 δ-Pu at elevated temperatures.

Plutonium has strong electronic correlations, which makes it difficult to establish microscopic understanding of the complicated structural phase diagram. Here the authors identify an electronic contribution to the entropy that stabilises the δ-Pu phase.

A unified and efficient theory for the structural properties of actinides and phases of plutonium

We show that a calculation using density functional theory (DFT) in the generalized gradient approximation (GGA) supplemented by an explicit Coulomb interaction term between correlated electrons (GGA+U), can accurately describe structural properties of (1) the room temperature phases of U, Np, Pu, Am and Cm, and (2) the α, β, γ, δ and ϵ phases of plutonium, as does the combination of GGA with dynamical mean field theory (DMFT). It thus changes the view on the role of electronic interaction in these systems and opens the way to fast calculations of structural properties in actinides metallic system. We use ab initio values of effective Coulomb interactions and underline that Hund's exchange and spin-orbit coupling are of utmost importance in these calculations. Secondly, we show that phonons properties in δ plutonium are impacted by strong interactions. The GGA+DMFT results exhibits a lattice instability for the transverse (1 1 1) phonon mode. Moreover the amplitude of this lattice instability is consistent with the experimental temperature of stability of this phase. Our calculation thus shows that when the δ phase is thermodynamically unstable (at 0 K), it is also dynamically unstable.