Nature of the 5$f$ electronic structure of plutonium

TL;DR

This study comprehensively analyzes the 5f electronic structure of all six plutonium allotropes using advanced theoretical methods, revealing their mixed-valence metallic nature and orbital-dependent correlations.

Contribution

It provides the first detailed theoretical characterization of the electronic structures of all six Pu allotropes, highlighting their mixed-valence behavior and orbital-dependent correlations.

Findings

All six allotropes are mixed-valence metals with atomic eigenstate fluctuations.

The 5f occupancy varies between 5.1 and 5.4 depending on volume and correlation.

The 5f electrons in the δ' phase are the most correlated and localized.

Abstract

Plutonium (Pu), in which the 5$f$ valence electrons always wander the boundary between localized and itinerant states, exhibits quite complex crystal structures and unprecedentedly anomalous properties with respect to temperature and alloying. Understanding its chemical and physical properties, especially its 5$f$ electronic structure is one of the central and unsolved topics in condensed matter theory. In the present work, the electronic structures of the six allotropes of Pu (including its $\alpha$, $\beta$, $\gamma$, $\delta$, $\delta'$, and $\epsilon$ phases) at ambient pressure are studied comprehensively by means of the density functional theory in combination with the single-site dynamical mean-field theory. The band structures, total and partial density of states, valence state histograms, 5$f$ orbital occupancies, X-ray branching ratios, and self-energy functions are carefully studied. It is suggested that the $\alpha$, $\beta$, and $\gamma$ phases of Pu are typical Racah metals in which the atomic multiple effect dominates near the Fermi level. The calculated results reveal that not only the $\delta$ phase, but also all the six allotropes are archetypal mixed-valence metals with remarkable atomic eigenstate fluctuation. In consequence of that, the 5$f$ occupancy $n_{5f}$ is around 5.1 $\sim$ 5.4, which varies with respect to the atomic volume and electronic correlation strength of Pu. The 5$f$ electronic correlation in Pu is moderately orbital-dependent. Moreover, the 5$f$ electrons in the $\delta'$ phase are the most correlated and localized.