Pu is arguably the most complex elemental metal in the periodic table because its 5f electrons are tenuously poised at the edge between localized and itinerant configurations. Metallic δ-Pu exhibits a Pauli-like magnetic susceptibility, electrical resistivity, and a Sommerfeld coefficient of the specific heat that are an order of magnitude larger than in any other elemental metal. In this talk, I highlight our inelastic neutron and nuclear magnetic resonance (NMR) spectroscopy used to understand the physics and chemistry of plutonium and Pu compounds. Prof. Albrecht-Schmitt played a critical role in several of our studies of Pu compounds over the years and has been a cheerleader for many others. We investigated the magnetic fluctuation spectrum of δ-Pu via inelastic neutron scattering using the ARCS spectrometer at the Spallation Neutron Source at Oak Ridge National Laboratory [1]. Our study reveals that the ground state of plutonium is governed by valence fluctuations, that is, a quantum-mechanical admixture of distinct 5f electronic configurations. Our results, which are in quantitative agreement with dynamical mean field theory calculations, show that the magnetism in Pu is not "missing" but is dynamic and driven by virtual valence fluctuations. Our measurements provide a straightforward interpretation of the microscopic origin of the large, Pauli-like magnetic susceptibility of δ-Pu. Nuclear magnetic resonance spectroscopy has developed into a cornerstone of chemistry and medicine for determination of the structure of chemical and biological systems through spin-1/2 1H and 13C NMR, and for imaging the body. The spin-1/2, 239Pu NMR signal was only recently observed in cubic PuO2, despite more than 50 years of effort on a range of 239Pu compounds [2]. We report the second observation of a Pu-239 NMR signal in a tetragonal compound PuB4. This compound is predicted to be a topological insulator [3] and measurements of the NMR shift reveal a significant difference (~25%) in the shift between PuO2 and PuB4, suggesting large changes in bonding in the two materials. Our 2nd observation of the Pu-239 NMR signal in PuB4 opens up a new window into understanding chemical bonding, aging, and correlated electron phenomena in Pu and its compounds [4].

Plutonium physics and chemistry highlights from Los Alamos National Laboratory: Neutron and NMR spectroscopy

2019 Spring National Meeting

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