Probing the population of the spin–orbit split levels in the actinide 5f states

Abstract

Spin-orbit interaction in the 5f states is believed to strongly influence exotic behaviors observed in actinides metals and compounds. Understanding these interactions and how they relate to the actinide series is of considerable importance. To address this issue, the branching ratio of the white-line peaks of the N 4,5 edges for the light actinide metals, -Th, -U, and -Pu were recorded using electron energy-loss spectroscopy (EELS) in a transmission electron microscope (TEM) and synchrotron-radiation-based x-ray absorption spectroscopy (XAS). Using the spin-orbit sum rule and the branching ratios from both experimental spectra and many-electron atomic spectral calculations, accurate values of the spin-orbit interaction, and thus the relative occupation of the j = 5/2 and 7/2 levels, are determined for the actinide 5f states. Results show that the spin-orbit sum rule works very well with both EELS and XAS spectra, needing little or no correction. This is important, since the high spatial resolution of a TEM can be used to overcome the problems of single crystal growth often encountered with actinide metals, allowing acquisition of EELS spectra, and subsequent spin-orbit analysis, from nm-sized regions.The relative occupation numbers obtained by our method have been compared with recent theoretical results and show a good agreement in their trend.

Nanoscale heterogeneity, premartensitic nucleation, and a new plutonium structure in metastableδfcc Pu-Ga alloys

“…The elastic response and the associated structural transformations-both in the phase diagram and the local ones described here-are extreme because of the narrowness of the f bands and the fact that the f electrons are ambivalent, occupying both localized and itinerant bonding states [11,22] with the former favored at Pu because of a strong spin-orbit interaction [10,98,99]. These characteristics contribute to the unusually strong electron-phonon coupling [9] that is the origin of the sensitivity of Pu and its alloys to transformation [55].…”

Nature of the5fstates in actinide metals

“…The incident energy of the TEM electron source used in these studies is 297 keV, ensuring the electron transitions are close to the electric-dipole limit. The use of apertures that remove high-angle Bragg and plural scattering (Moore et al, 1999a;1999b;2002b) further refine the quality of EELS, providing spectra that are practically identical to XAS for transition metals (Blanche et al, 1993), rare earths (Moore et al, 2004b), and actinides (Moore et al, 2006c). Looking specifically at actinides, direct comparison of Th and U O 4,5 edge can be made between EELS (Moore et al, 2003;2004a) and XAS (For Th: Cukier et al, 1978;Aono et al, 1981;for U: Cukier et al, 1978;Iwan et al, 1981).…”

“…In our EELS and many-electron atomic spectral calculations we observe either a 0.6 or 1.3 5f electrons, depending on the type of background removal utilized. In van der Laan et al (2004) and Moore et al (2004a;2006c) a standard XAS background removal and peak fitting is used, yielding n f = 0.6. This is of course close to the 0.5 count given by Baer and Lang (1980).…”

Self-interaction-corrected local spin density theory of5felectron localization in actinides

“…The electronic structure of actinides remains an active area of research both experimentally 1,2,3,4,5,6,7,8,9,10,11,12 and theoretically. 13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31 The intricate nature of the f -electrons, their sensitivity to external probes, like temperature and pressure, and their magnetic ordering are at the center of interest.…”