Theoretical Modeling of f Electron Systems

Monday, November 10, 2014 2:00 PM in Room 301

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2:00 PM AC+AS+MI+SA+SS-MoA-1 Nonmagnetic Ground State of PuO2
Jindrich Kolorenc (Academy of Sciences of the Czech Republic)
The correlated band theory implemented as a combination of the local density approximation with the dynamical mean-field theory is applied to PuO2. We obtain an insulating electronic structure consistent with the experimental photoemission spectra, and a nonmagnetic ground state that is characterized by a noninteger filling of the plutonium f shell (nf ≈ 4.4). Due to a sizable hybridization of the f shell with the p states of oxygen, the ground state is more complex than the four-electron Russell–Saunders 5I4 manifold split by the crystal field. The dynamical mean-field theory, which in the present case can be schematically viewed as an extension of the crystal-field model with hybridization terms, improves the agreement between the theory and experiment for the magnetic susceptibility [1]. Encouraged by the good accuracy achieved for PuO2 , we apply the theory to several other tetravalent actinide oxides.

[1] A. B. Shick, J. Kolorenč, L. Havela, T. Gouder, and R. Caciuffo, Phys. Rev. B 89, 041109 (2014).

2:40 PM AC+AS+MI+SA+SS-MoA-3 DMFT Modeling of Electronic Spectral Properties in Pu-based Actinides
Jian-Xin Zhu (Los Alamos National Laboratory)

Plutonium-based materials have been studied for many years due to their importance in nuclear energy applications. Scientifically, these materials exhibit highly complex properties. Pu metal shows a significant volume expansion and anomalous magnetic properties; while Pu-115 (like PuCoGa5) are found to be superconductors. These intriguing phenomena originate from the special location of Pu in the Periodic Table, which is at the boundary between the light actinides that have itinerant 5f electrons and the heavy actinides with localized 5f electrons. They call out the notion of strong correlation of 5f electrons. In this talk, I will present a study of the electronic structure of Pu metals and its 115 compounds in the framework of the combination of local density functional approximation and dynamical mean-field theory. In particular, the results on momentum-resolved spectral functions will be presented and be compared with those based on the LDA only. In addition, the effect of Pu 5f electron occupancy on the electronic structure of these systems will also be discussed. The test of these results by future angle-resolved photoemission spectroscopy measurements will give a stringent constraint on the theoretical approach.

3:40 PM AC+AS+MI+SA+SS-MoA-6 The Evolution in Pu Nanocluster Electronic Structure: From Atomicity to Three-Dimensionality
James Tobin, Sung-Woo Yu, Brandon Chung (Lawrence Livermore National Laboratory); Mikhail Ryzhkov (Russian Academy of Science-Urals); Alexey Mirmelstein (Russian Federation Nuclear Lab (VNIITF))

The development of electronic structure in solid systems as a function of size has long been a subject of great interest and extensive scientific investigation. Experimentally, the transition, from nanoscale or mesoscopic to bulk behavior in metal clusters, was reported in 1981 by Mason and co-workers. Similarly, the evolution from two-dimensional to three-dimensional band structure in metal overlayers and the manifestation of nanoscale effects in compound semiconductor have also observed. In the area of actinide materials, the progress has been slowed by the limitations imposed by the highly radioactive, chemically toxic and pyrolytic nature of these materials. Havela and Gouder and colleagues performed investigations upon Plutonium (Pu) ultra-thin films, deposited in situ by means of a discharge-plasma, and Trelenberg and co-workers developed an approach using laser ablation of Uranium (U). Gas phase studies of actinides have also been pursued including atoms, molecules and reactions. Recent theoretical studies include UO2 molecules, solid actinide oxides, and actinide carbide clusters. A new approach to cluster calculations has been taken in this study. Past cluster calculations were arranged in such a way that the central atom would exist in a bulk like environment. In calculations herein, it is expected that the central atom will be in the most bulk-like environment as well. However, just as in any finite size object, there will be variation of potential at the positions of symmetry non-equivalent atoms within the simulated cluster. Hence, averaging over all of the atoms in the cluster will give a measure of the effect of size. We will use this aspect of cluster calculations to investigate size related effects. Here, we report the result of the calculation of the electronic structure of clusters of Pu and their comparison to bulk spectroscopic results. Lawrence Livermore National Laboratory is operated by Lawrence Livermore National Security, LLC, for the U.S. Department of Energy, National Nuclear Security Administration under Contract No. DE-AC52- 07NA27344. Work at the RAS and VNIITF was supported in part by Contract B590089 between LLNL and VNIITF. The Advanced Light Source (ALS) in Berkeley and the Stanford Synchrotron Radiation Laboratory are supported

by the DOE Office of Science, Office of Basic Energy Science. For more detail see:M.V. Ryzhkov, A. Mirmelstein, S.-W. Yu, B.W. Chung and J.G. Tobin, “Probing Actinide Electronic Structure through Pu Cluster Calculations,” Intl. J. Quantum Chem. 113, 1957 (2013); COVER ARTICLE .

4:00 PM AC+AS+MI+SA+SS-MoA-7 First-Principles Density Functional Theory Simulation on Rare-Earth-Based Oxides as Fast Oxygen Ion Conductors
Mamoru Sakaue, Musa Alaydrus, Hideaki Kasai (Osaka University, Japan); Tatsumi Ishihara (Kyushu University, Japan)
Development of novel fast ion conductors is a crucial issue for realizing solid oxide fuel cells (SOFCs) which can operate in low temperatures. While yttria-stabilized zirconia (YSZ) had been well-studied both by experiments and theories, exploration of other types of materials retaining high ionic conductivities in lower temperatures is still desired. Experimental studies in the recent twenty years have found some rare-earth-based oxides having higher ionic conductivities to be promising for operation below 600°C, and first-principles simulation studies on the materials have begun.
We studied atomic and electronic properties on oxygen-ionic conduction of LaGaO3-, La2GeO5-, Pr2NiO4- and CeO2-based materials by first-principles calculations based on density functional theory (DFT). We analyzed stable structures, electronic densities of states, oxygen migration paths and activation energies in the paths of pure and doped materials in order to evaluate their capabilities in application to electrolytes or electrodes. The obtained results of the activation energies showed good agreements with experiments in several aspects. However, for Pr2NiO4- and CeO2-based materials that contain lanthanoid elements, there remains an open question about theoretical treatment of 4f electron states. The strong localization was found to affect oxygen ion motions fundamentally as well as electronic/magnetic properties. Then the strong electron-electron correlation modifies simple trends in activation energies found for the lanthanoid series within an approximation in which the correlation effects were neglected [1].
While fundamental reproduction of the strong electron-electron correlation in localized states by DFT is extremely difficult, empirical corrections by Hubbard U terms enable a practical solution to this problem. Here, the U value can be determined only by comparison with experiments in most cases because the value is affected by environment of the lanthanoid atoms. However, a theoretical study based on an empirical model demonstrated that the environment effects on 4f orbitals can be small if its strong localization is maintained [2]. In the presentation, we confirm this rule based on the results by the DFT first-principles calculations with Hubbard U corrections and discuss possibilities of computational materials design of lanthanoid-doped ceria (Ce1-xLnxO2-x/2) as electrolyte materials.
[1] M. Alaydrus, M. Sakaue, S. M. Aspera, T. D. K. Wungu, T. P. T. Linh, H. Kasai, T. Ishihara and T. Mohri, J. Phys. Condens. Matter 25, 225401 (2013).
[2] E. Rogers, P. Dorenbos and E. van der Kolk, New J. Phys. 13, 093038 (2011).
4:20 PM AC+AS+MI+SA+SS-MoA-8 Electronic Structure, Magnetic Properties, and Magneto-Structural Transformations of Rare Earth Magneto-Caloric Materials
Durga Paudyal (Ames Laboratory); Vitalij Pecharsky, Karl Gschneidner, Jr. (Ames Laboratory and Iowa State University)
We present first principles modeling of structural and magnetic properties of Gd5Ge4 based magneto-caloric materials. The total energy as a function of the shear displacement of slabs confirms stability of experimentally observed crystal and magnetic structures. Small substitutions of the Gd by Y and Lu lead to a catastrophic loss of ferromagnetism, but the substitutions by La have no effect on the magnetism. Furthermore, substitutions of the Ge by Si exert chemical pressure and transform the antiferromagnetic O(II) to the ferromagnetic O(I) ground state. In addition, we present a pathway for estimating the magnetic entropy change in the room temperature giant magnetocaloric compounds, i.e. Gd5Si2Ge2, by coupling first principles outputs with the established magneto-thermodynamic models. The theoretical values of the magnetic entropy change compare well with experimental results.

This work was supported by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences, Materials Science and Engineering Division. The research was performed at the Ames Laboratory, which is operated for the U.S. DOE by Iowa State University under contract # DE-AC02-07CH11358.