Fluctuating valence in a correlated solid and the anomalous properties of δ-plutonium

Shim, Jun Ho; Haule, Kristjan; Kotliar, Gabriel

doi:10.1038/nature05647

Cited by 284 publications

(366 citation statements)

References 34 publications

Supporting

Mentioning

332

Contrasting

Order By: Relevance

“…Most of the previous works have concentrated on molecular dynamic simulations with empirically adjusted interatomic potentials [2] [3]. However, the 5f electrons in actinides are close to a localization-delocalization or Mott transition as it has been recently demonstrated for Plutonium metal [4][5] [6]. UO 2 and PuO 2 are Mott-Hubbard insulators with energy gaps at both low and high temperatures.…”

mentioning

confidence: 99%

“…Here, the 5f -electron self-energies extracted from the cluster exact diagonalization are subsequently fit using three-pole interpolation. An effective parameter U ef f = 3eV describing the on-site Coulomb repulsion among the 5f electrons is used while the other Slater integrals (F (2) , F (4) and F (6) ) are computed from atomic physics, and are subsequently rescaled to 80% of their values to account for the effect of screening [18].…”

mentioning

confidence: 99%

See 1 more Smart Citation

Origin of Low Thermal Conductivity in Nuclear Fuels

2008

View full text Add to dashboard Cite

Using a novel many-body approach, we report lattice dynamical properties of UO2 and PuO2 and uncover various contributions to their thermal conductivities. Via calculated Grüneisen constants, we show that only longitudinal acoustic modes having large phonon group velocities are efficient heat carriers. Despite the fact that some optical modes also show their velocities which are extremely large, they do not participate in the heat transfer due to their unusual anharmonicity. Ways to improve thermal conductivity in these materials are discussed.Today's nuclear fuels are based on 235 U and 239 Pu elements where in a typical set-up, a nuclear reaction heats up a pellet made of either UO 2 or its mixture with PuO 2 and the heat is transformed to electrical energy. One of the major issues is to conduct the heat from a core of the pellet to its outer area which makes the evaluation of a high temperature thermal conductivity a key problem. Unfortunately, the thermal conductivity of UO 2 is very low and as a result, even a gradual increase of its value may lead to significant breakthrough in the performance of commercial nuclear reactors currently operating worldwide.In the present work, we argue that it is the unexpectedly large anharmonicity of optical modes that results in a very low thermal conductivity of modern nuclear fuels. Consider semiconducting UO 2 which is a main element of UOX fuel, or a blend of U and Pu oxides which is a major substance of MOX fuel. The heat from the core of the pellet in these insulating systems is carried by phonons which are known to be very inefficient heat conductors. This brings a whole set of complex problems such as causing fuel pellets to crack and degrade prematurely, necessitating replacement before the fuel has been depleted.Unfortunately, studying thermal conductivity In the present work we use a novel electronic structure method [13] capable of describing Mott insulating materials in order to address the structural properties and thermal conductivity of the modern nuclear fuels. Both UO 2 and PuO 2 are calculated using a combination of local density approximation (LDA) [12] and Dynamical Mean Field Theory (DMFT) [14][15] where relativistic 5f shells of Uranium and Plutonium atoms are treated by exact diagonalization of corresponding many-body Hamiltonians obtained by allowing a hybridization between the 5f -electrons and the nearest oxygen 2p orbitals.It is well known that a strong spin-orbit coupling of about 1eV present in actinides splits 14-fold degenerate f level onto f 5/2 and f 7/2 states. Group theoretical considerations assume that under cubic crystal symmetry, the f 5/2 6-fold degenerate level is further split onto Γ 8 quadruplet and Γ 7 doublet. In both UO 2 and PuO 2 , the Γ 8 level comes approximately 0.1eV below the Γ 7 state, and valence arguments make it occupied by two electrons for the case of UO 2 and fully occupied by four electrons for the case of PuO 2 . This sequence of the levels dictates the low temperature properties of these two materials: The UO ...

show abstract

mentioning

confidence: 99%

mentioning

confidence: 99%

Origin of Low Thermal Conductivity in Nuclear Fuels

2008

View full text Add to dashboard Cite

show abstract

“…These values suggest that the Pu-5f electronic states are in the mixed-valence regime. The Pu5f mixed-valence behavior together with the multiplet effects have also been proposed in the elemental Pu solid 26,27 . Therefore, this resemblance leads one to believe that the second possibility is more likely.…”

Section: Resultsmentioning

confidence: 99%

“…Recently, the LDA+DMFT method [21][22][23] , which is a combination of LDA with the dynamical mean-field theory (DMFT) 19,20 needed to capture important quantum dynamical fluctuation effects, has become a powerful tool to address the strongly correlated electronic materials. When applied to the δ-phase of Pu, the LDA+DMFT method can explain not only the large volume expansion 22,24 but can also address the absence of magnetism 25,26 and the quasiparticle peak near the Fermi energy [26][27][28][29][30][31] . The same approach has also recently demonstrated site-selective electronic correlation effects in the α-phase of Pu 32 .…”

Section: Introductionmentioning

confidence: 99%

First-principles study of the Kondo physics of a single Pu impurity in a Th host

et al. 2015

Self Cite

View full text Add to dashboard Cite

Based on its condensed-matter properties, crystal structure, and metallurgy, which includes a phase diagram with six allotropic phases, plutonium is one of the most complicated pure elements in its solid state. Its anomalous properties, which are indicative of a very strongly correlated state, are related to its special position in the periodic table, which is at the boundary between the light actinides that have itinerant 5f electrons and the heavy actinides that have localized 5f electrons. As a foundational study to probe the role of local electronic correlations in Pu, we use the local-density approximation together with a continuous-time quantum Monte Carlo simulation to investigate the electronic structure of a single Pu atom that is either substitutionally embedded in the bulk and or adsorbed on the surface of a Th host. This is a simpler case than the solid phases of Pu metal. For the Pu impurity atom we have found a Kondo resonance peak, which is an important signature of electronic correlations, in the local density of states around the Fermi energy. Furthermore, we show that the peak width of this resonance is narrower for Pu atoms at the surface of Th than for those in the bulk due to a weakened Pu 5f -ligand hybridization at the surface.

show abstract

“…3 imply that the preferred intermediate phase for the fcc (δ) → monoclinic (α, α ′ ) transformation in Pu is the hcp phase and the corresponding energy barrier is about 0.2 eV/atom. It is well-known that Pu has a unique position among the actinides that stems from the behavior of its f electrons [23,24]. In particular, the f electrons in early actinides (Ac to Np) are itinerant and participate in bonding, whereas in the late actinides (Pu to No) they are localized at individual atoms.…”

Section: Discussionmentioning

confidence: 99%

Atomistic studies of transformation pathways and energetics in plutonium

Gröger

Lookman

Saxena

2009

Philosophical Magazine

View full text Add to dashboard Cite

One of the most challenging problems in understanding the structural phase transformations in Pu is to determine the energetically favored, continuous atomic pathways from one crystal symmetry to another. This problem involves enumerating candidate pathways and studying their energetics to garner insight into instabilities and energy barriers. The purpose of this work is to investigate the energetics of two transformation pathways for the δ → α ′ transformation in Pu that were recently proposed [1] on the basis of symmetry. These pathways require the presence of either an intermediate hexagonal closed-packed (hcp) structure or a simple hexagonal (sh) structure. A subgroup of the parent fcc and the intermediate hexagonal structure, which has trigonal symmetry, facilitates the transformation to the intermediate hcp or sh structure. Phonons then break the translational symmetry from the intermediate hcp or sh structure to the final monoclinic symmetry of the α ′ structure. We perform simulations using the modified embedded atom method (MEAM) for Pu to investigate these candidate pathways. Our main conclusion is that the path via hcp is energetically favored and the volume change for both pathways essentially occurs in the second step of the transformation, i.e. from the intermediate sh or hcp to the monoclinic structure. Our work also highlights the deficiency of the current state-of-the-art MEAM potential in capturing the anisotropy associated with the lower symmetry monoclinic structure.

show abstract

Fluctuating valence in a correlated solid and the anomalous properties of δ-plutonium

Cited by 284 publications

References 34 publications

Origin of Low Thermal Conductivity in Nuclear Fuels

Origin of Low Thermal Conductivity in Nuclear Fuels

First-principles study of the Kondo physics of a single Pu impurity in a Th host

Atomistic studies of transformation pathways and energetics in plutonium

Contact Info

Product

Resources

About