Theoretical zero-temperature phase diagram for neptunium metal

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.