Plutonium: NMR examines electrons

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  • Published: Oct 1, 2016
  • Author: David Bradley
  • Channels: NMR Knowledge Base
thumbnail image: Plutonium: NMR examines electrons

Heavy metal

Using nuclear magnetic resonance spectroscopy in EMSL’s Rad Annex, Dr. Herman Cho and his team delved into the behavior of one of the most complex elements: plutonium. Image courtesy: PNNL

When it comes to plutonium, things are complicated. The radioactive element comes with a lot of technical and regulatory baggage. Now, a team at the Pacific Northwest National Laboratory and Washington State University have used fluorine-19 nuclear magnetic resonance (NMR) spectroscopy to take a close look at the element's inner electronic secrets when it finds itself in the superficially simple compound plutonium tetrafluoride.

Actinide elements have been viewed as 5f analogues of the 4f lanthanide series in the Periodic Table in terms of their electron structure, hence the placement of the actinide row below the lanthanide row. Indeed, the chemistry of the heavy actinides is akin to that of the lanthanides, but as far as plutonium is concerned there is something of a discontinuity, where we see the lighter actinides actually resembling the chemical properties of the transition metals rather than the weightier members of the row. Plutonium can therefore be seen as a pivot point in the periodic table. Understanding its electronic structure might help improve our understanding of the elements that lie to either side of it in the table.

Everybody needs good neighbours

Thus, what could be simpler? A metal ion surrounded by just four atoms of a non-metal element? But, the simple formula for plutonium tetrafluoride belies a complexity that is only now revealing itself. One might imagine that unlike a conventional salt, say sodium chloride (a metal and a non-metal), the bonds would be covalent in the plutonium compound. However, little electron-sharing covalency is observed. Even though the plutonium and fluorine atoms are tied together in a lattice, they act as isolated atoms drawn by electrostatic forces. The nature of this interaction is one of the big questions for plutonium and its actinide neighbours in the Periodic Table; as team leader Herman Cho points out Answering this question is of huge importance because plutonium's chemistry depends on how it bonds. Plutonium tetrafluoride leans toward electrostatic attraction. This work provides a clearer picture of why that is," he says.

Plutonium has a formidable cloud of electrons surrounding its nucleus and as such it does not always behave like a lesser metal. Of course, research into this element is difficult not only for scientific reasons but also regulatory, due to the manifold safety and security concerns associated with plutonium. There are, understandably, only a limited number of centres that can safely handle and study the radioactive element. The PNNL research sheds new light on the ways in which plutonium interacts with neighbouring atoms and their findings could offer invaluable insights for nuclear power, international security, and environmental issues associated with plutonium.

On to neptunium

The researchers employed NMR instruments at the Radiochemical Processing Laboratory at PNNL and the Rad Annex of the US Department of Energy's EMSL, a national scientific user facility. These facilities are two of the few in the world that can perform NMR measurements on plutonium-containing solids. The NMR spectra revealed that both the plutonium and fluorine atoms cling to their atoms as they would if the material was completely ionic rather than molecular. Specifically, the team probed the magnetic moment of polycrystalline plutonium tetrafluoride acquiring spectra at applied magnetic fields of 2.35 and 7.05 Tesla.

"Plutonium's lighter neighbours in the actinide row - thorium, uranium, and neptunium - as well as counterparts in the parallel lanthanide row above it, form tetrafluorides with almost identical structures," Cho told us. "These compounds thus offer valuable comparisons with plutonium tetrafluoride that would further elucidate the nature and trends of the metal-ligand interactions. Spectroscopic experiments at low temperatures would allow electron correlations to be observed and studied; such correlations are at the center of many of the unique properties associated with heavy elements, and measurements of magnetic parameters would provide an informative view of the factors governing the formation of electron correlations."

Related Links

Phys Rev B 2016, 93(22) 224409: "Probing the Pu4+ magnetic moment in PuF4 with 19F NMR spectroscopy"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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