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Site-Selective Electronic Correlation in Α-Plutonium Metal

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Site-Selective Electronic Correlation in Α-Plutonium Metal ARTICLE Received 18 Jun 2013 | Accepted 19 Sep 2013 | Published 18 Oct 2013 DOI: 10.1038/ncomms3644 Site-selective electronic correlation in a-plutonium metal Jian-Xin Zhu1, R.C. Albers1, K. Haule2, G. Kotliar2 & J.M. Wills1 An understanding of the phase diagram of elemental plutonium (Pu) must include both, the effects of the strong directional bonding and the high density of states of the Pu 5f electrons, as well as how that bonding weakens under the influence of strong electronic correlations. Here we present electronic-structure calculations of the full 16-atom per unit cell a-phase structure within the framework of density functional theory together with dynamical mean- field theory. Our calculations demonstrate that Pu atoms sitting on different sites within the a-Pu crystal structure have a strongly varying site dependence of the localization– delocalization correlation effects of their 5f electrons and a corresponding effect on the bonding and electronic properties of this complicated metal. In short, a-Pu has the capacity to simultaneously have multiple degrees of electron localization/delocalization of Pu 5f electrons within a pure single-element material. 1 Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA. 2 Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08854, USA. Correspondence and requests for materials should be addressed to J.-X.Z. (email: [email protected]). NATURE COMMUNICATIONS | 4:2644 | DOI: 10.1038/ncomms3644 | www.nature.com/naturecommunications 1 & 2013 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms3644 ure plutonium has long been considered to be one of the strongly localized. Our calculations demonstrate that this is most exotic elemental solids in the periodic table with caused by the effects of dissimilar near-neighbor atomic Prespect to its very unusual physics properties, crystal coordinations for the different sites on the hybridization strengths structure and metallurgy1–6, including a phase diagram with six of the Pu 5f electrons to the other conduction electrons, which are allotropic phases, with the low-temperature monoclinic a-phase subsequently reflected in the effects of correlation on the site- stable up to 395 K and the technologically important face-center- projected densities of states. These results show that, if the cubic (fcc) d-phase stable between 592 K and 724 K. Importantly, element Pu is a ‘crossover’ element, one might consider the for practical purposes, small amounts of other alloying elements a-phase as a ‘crossover’ phase, with mixed electronic correlation have been shown to stabilize the d-phase at room temperature. In among its different atoms that eventually locks into the d-phase, both the pure material and also in these alloys the d-phase where every Pu atom is strongly correlated. transformation of Pu exhibits a significant atomic volume expansion (B25%) relative to the a-phase. This d-phase has a variety of unusual and anomalous physical and mechanical Results properties3 that are believed to be caused by a change of the 5f Crystal structure and bond lengths. So far, almost all of the electronic orbitals from having an itinerant (metallic) to a more sophisticated recent calculations have been done for pre- localized behaviour. This occurs for plutonium, as this element is dominantly the fcc d-phase, because this crystal structure has at the boundary between the light actinides that have itinerant 5f only one Pu atom per unit cell and is therefore amenable to better electrons and the heavy actinides with localized 5f electrons4. The but more computationally intensive techniques (such as LDA þ actinides are the only materials in the periodic table that exhibit DMFT) to handle the electronic correlations. However, the this type of crossover in the middle of a row. equilibrium phase at room temperature and below is actually the The d-phase of Pu has frequently been an exciting topic of a-phase of Pu. This phase has a long history of controversy. Its discussion within the condensed matter physics community, as it crystal structure is very complex. As shown in Fig. 1, a-Pu has a has a simple fcc crystal structure and strong electronic correlation low symmetry monoclinic structure with space group P21/m.It effects, which quench the expected magnetism that is typically involves 16 atoms per unit cell with eight unique atomic sites30,31. expected for narrow-band materials. The simplest theoretical At 294 K, the unit cell has dimensions of a ¼ 6.183 Å, b ¼ 4.822 Å, treatment, band-structure calculations within the local density c ¼ 10.963 Å and b ¼ 101.79°. All the atoms are located at either approximation (LDA) or the generalized gradient approximation (x, 1/4, z)or(À x, 3/4, À z) positions, but they have a very (GGA), fails to predict the equilibrium volume of the non- complicated and distorted nearest-neighbor arrangement of magnetic d-phase of Pu7,8 due to the strong electronic Pu–Pu bonds, as we shall describe below. A simple intuitive correlations in Pu. Several research groups have therefore picture can be derived from a tight-binding description, which is applied the LDA þ U method in order to include more f–f natural for a narrow-band system like Pu. As the f electrons correlation energy. By adjusting the on-site Coulomb repulsion dominate the bonding properties, one can focus mainly on the energy (U) appropriately, it has been possible to fit to the coupling between these f-electron states. If one examines the experimental d-phase volume. However, earlier LDA þ U individual f-electron orbitals, one discovers that they have a very calculations also indicated an instability of d-Pu toward an complicated arrangement of lobes pointing in many different antiferromagnetic ground state9–11. Later calculations based on directions. From this picture, it is not surprising that any attempt the ‘fully localized-limit’12 or ‘around-mean-field’13–15 LSDA þ U to form the atoms into a periodic array would have difficulty with the spin–orbit interaction method were able to obtain the optimizing the overlap of these various f orbitals. The resulting experimentally observed quenching of the spin and orbital magnetism by using a closed-shell Pu 5f6 configuration as well as the correct d-phase volume. However, due to its inherently Pu VII static treatment of electronic correlation, the LDA þ U approach Pu I fails to reproduce a major 5f-character quasiparticle peak near Pu VIII the Fermi energy as observed by photoemission spectroscopy on Pu VIII 16,17 d-Pu . To reproduce the spectral properties, in addition to Pu I other thermodynamical ones, improved approaches are needed to Pu VII capture the quantum fluctuation effects of electronic correlation. Pu II Pu III Pu V Pu IV The dynamical mean-field theory (DMFT) is a many-body Pu VIII Pu II technique that enables the treatments of band- and atomic-like 18,19 Pu VI Pu VI aspects of electronic states on the same footing . Merging this Pu III 20,21 with LDA-based methods, LDA þ DMFT method provides a Pu V Pu V Pu III Pu IV Pu III new calculation scheme for strongly correlated electronic Pu III Pu IV materials. Within the LDA þ DMFT approach21,22, the origin of substantial volume expansion of d-Pu has been explained in Pu VI terms of a competition between Coulomb repulsion and kinetic Pu VI Pu V Pu VIII energy. Later LDA þ DMFT calculations with various impurity Pu IV Pu III Pu II Pu II solvers have not only reaffirmed the absence of magnetism in 23,24 d-Pu but have also reproduced the quasiparticle peak near the Pu I Pu VII Fermi energy24–29. These intensive studies have established firmly Pu VIII the dual character of the 5f electronic states in Pu. Pu VIII Although much attention has been focused on d-Pu (for reasons given above), in this Communication we show that a-Pu c Pu I a Pu VII is perhaps even more interesting. Surprisingly, for a pure single- b element material, our LDA þ DMFT calculations show that different Pu atoms in a-Pu have different amounts of electronic Figure 1 | Crystal structure of a-Pu. The 16 atoms are grouped into 8 correlation effects that range from itinerant-like to much more nonequivalent atomic types. 2 NATURE COMMUNICATIONS | 4:2644 | DOI: 10.1038/ncomms3644 | www.nature.com/naturecommunications & 2013 Macmillan Publishers Limited. All rights reserved. NATURE COMMUNICATIONS | DOI: 10.1038/ncomms3644 ARTICLE highly distorted crystal structure appears to be basically some sort Pu split into groups of short (between 2.57–2.78 Å) and long of frustrated system of overlaps. The resulting arrangement of (between 3.19–3.71 Å) nearest-neighbor bond lengths. From atoms and crystal structure can be calculated and predicted very Fig. 2 it is quickly evident that the Pu sites can roughly be accurately by full-potential modern first-principle LDA-like divided into three categories: out of eight unique Pu sites, six sites band-structure calculations, which automatically finds the lowest (sites II–VII) have similar types of bonding, with 4 short nearest- total energy (maximum overlap). Based on this success, it has neighbor bonds and 10 long bonds. However, two other sites are been suggested that there is very little correlation for a-Pu, qualitatively different. Site-I has five short bonds with three of because standard band-structure calculations have minimal cor- them at the shortest of any Pu–Pu atomic distance and seven long relation. Nonetheless, it was noted quite early on that the volume bonds, whereas site VIII has only three short bonds with all of per atom plots of the actinide series as a function of atomic them at the longest distances in the short-bond range and 13 long number showed an upturn for a-Pu instead of continuing bonds.
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