K. T. Moore

Nature of the 5f states in actinide metals

Actinide elements produce a plethora of interesting physical behaviors due to the 5f states. This review compiles and analyzes progress in understanding of the electronic and magnetic structure of the 5f states in actinide metals. Particular interest is given to electron energy-loss spectroscopy and many-electron atomic spectral calculations, since there is now an appreciable library of core d -> valence f transitions for Th, U, Np, Pu, Am, and Cm. These results are interwoven and discussed against published experimental data, such as x-ray photoemission and absorption spectroscopy, transport measurements, and electron, x-ray, and neutron diffraction, as well as theoretical results, such as density-functional theory and dynamical mean-field theory.

X-ray and electron microscopy of actinide materials

Actinide materials demonstrate a wide variety of interesting physical properties in both bulk and nanoscale form. To better understand these materials, a broad array of microscopy techniques have been employed, including transmission electron microscopy (TEM), electron energy-loss spectroscopy (EELS), energy dispersive X-ray spectroscopy (EDXS), high-angle annular dark-field imaging (HAADF), scanning electron microscopy (SEM), wavelength dispersive X-ray spectroscopy (WDXS), electron back scattered diffraction (EBSD), scanning tunneling microscopy (STM), atomic force microscopy (AFM), and scanning transmission X-ray microscopy (STXM). Here these techniques will be reviewed, highlighting advances made in the physics, materials science, chemistry, and biology of actinide materials through microscopy. Construction of a spin-polarized TEM will be discussed, considering its potential for examining the nanoscale magnetic structure of actinides as well as broader materials and devices, such as those for computational magnetic memory.

On the electronic configuration in Pu: spectroscopy and theory

Photoelectron spectroscopy, synchrotron-radiation-based x-ray absorption, electron energy loss spectroscopy, and density-functional calculations within the mixed-level and magnetic models, together with canonical band theory, have been used to study the electron configuration in Pu. These methods suggest a 5fn occupation for Pu of 5≤n<6, with , contrary to what has recently been suggested in several publications. We show that the n = 6 picture is inconsistent with the usual interpretation of photoemission, x-ray absorption, and electron energy loss spectra. Instead, these spectra support the traditional conjecture of a 5f5 occupation in Pu as is obtained by density-functional theory. We further argue, based on 5f-band filling, that an n = 6 hypothesis is incompatible with the position of Pu in the actinide series and its monoclinic ground-state phase.

Probing electronic correlations in actinide materials using multipolar transitions

We report nonresonant inelastic x-ray scattering from the semicore 5d levels of several actinide compounds. Dipole-forbidden, high-multipole features form a rich bound-state spectrum dependent on valence electron configuration and spin-orbit and Coulomb interactions. Cross-material comparisons, together with the anomalously high Coulomb screening required for agreement between atomic-multiplet theory and experiment, demonstrate sensitivity to the neighboring electronic environment, such as is needed to address longstanding questions of electronic localization and bonding in 5f compounds.

The degree of 5f electron localization in URu2Si2: electron energy-loss spectroscopy and spin-orbit sum rule analysis

We examine the degree of 5f electron localization in URu{sub 2}Si{sub 2} using spin-orbit sum rule analysis of the U N{sub 4,5} (4d {yields} 5f) edge. When compared to {alpha}-U metal, US, USe, and UTe, which have increasing localization of the 5f states, we find that the 5f states of URu{sub 2}Si{sub 2} are more localized, although not entirely. Spin-orbit analysis shows that intermediate coupling is the correct angular momentum coupling mechanism for URu{sub 2}Si{sub 2} when the 5f electron count is between 2.6 and 2.8. These results have direct ramifications for theoretical assessment of the hidden order state of URu{sub 2}Si{sub 2}, where the degree of localization of the 5f electrons and their contribution to the Fermi surface are critical.

Electron-energy-loss spectroscopy and X-ray absorption spectroscopy as complementary probes for complex f-electron metals: cerium and plutonium

In this paper, we demonstrate the power of electron-energy-loss spectroscopy (EELS) in a transmission electron microscope by investigating the electron structure of two f-electron metals: Ce and Pu. It is shown that EELS in a transmission electron microscope may be used to circumvent the difficulty of producing single-phase or single-crystal samples owing to its high spatial resolution, and that diffraction patterns and images can be acquired, providing unambiguous phase determination when acquiring spectra. EELS results are supported by synchrotron-radiation-based X-ray absorption, multielectron atomic spectral simulations, and local density approximation calculations based on density-functional theory with the generalized gradient approximation. For Ce, it is shown that changes in {111} stacking sequences can drive substantial modifications in the electronic structure of close-packed phases of Ce that have similar atomic volumes, contrary to previous assumptions in literature. For Pu, it is shown that Russell–Saunders (L–S) coupling fails for the 5f states and that either a j–j or an intermediate scheme must be used for the actinides because of the considerable spin–orbit interaction in the 5f states. We present a model showing how the 5f states behave along the light actinide series.

Transmission electron microscopy investigation of an ordered metastable phase in Zr-N alloys

Supersaturated hcp f-Zr(N) alloys containing 22-28 at.% N were prepared by nitriding sheets of Zr in an atmosphere of high-purity N 2 , followed by homogenization under high-purity Ar gas. Quenching and isothermal ageing of the alloys for various times between 500 and 650C resulted in precipitation of a metastable phase, rather than the equilibrium phase ZrN. This investigation focused on determining the structure, orientation relationship, habit plane, morphology, growth kinetics and atomic growth mechanism of this non-equilibrium precipitate using transmission electron microscopy imaging and diffraction techniques, high-resolution transmission electron microscopy, electron-energy-loss spectroscopy and various simulation programs. The precipitate, which was arbitrarily designated the ξ phase, has a monoclinic Bravais lattice. Its lattice parameters are a = 0.32 nm, b = 0.60 nm, c = 0.56 nm, f= g= 90 and n= 121.5. Its orientation relationship with the fmatrix is

L1 Report for the Enhanced Surveillance Campaign Experimental Benchmarking of Pu Electronic Structure

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