Physicists Create Unexpected New Form of Plutonium

Physicists using the European Synchrotron Radiation Facility have created a new compound of plutonium (Pu) with an unexpected, pentavalent oxidation state — Pu (V). The new compound is solid and stable, and may represent a transient phase in radioactive waste repositories.

A plutonium dioxide nanoparticle. Image credit: Kristina Kvashnina.

A plutonium dioxide nanoparticle. Image credit: Kristina Kvashnina.

One of the most fundamental properties of the chemical behavior of Pu is the variety of its oxidation states (the oxidation state is defined by the number of electrons that are removed from the valence orbitals of a neutral atom).

Four oxidation states (from III to VI) may co-exist under environmental conditions, (VII) and even (VIII) states are proposed to be stable under highly alkaline oxidative conditions.

Pu in the pentavalent oxidation state, Pu (V), has three electrons in the 5f shell, leaving the 6d orbitals empty.

“It all started when we were trying to create plutonium dioxide nanoparticles using different precursors,” said Dr. Kristina Kvashnina, a physicist at the Helmholtz Zentrum Dresden-Rossendorf and based at the ROBL beamline at the European Synchrotron, and her colleagues.

When the researchers used the Pu (VI) precursor, they realized that a strange reaction took place during the formation of the plutonium dioxide nanoparticles.

“Every time we create nanoparticles from the other precursors, Pu(III), (IV) or (V), the reaction is very quick, but here we observed a weird phenomenon half way,” Dr. Kvashnina explained.

“We figured that it must be Pu (V), pentavalent plutonium, a never-observed-before form of the element, after doing a high-energy resolution fluorescence detection experiment.”

The further experiments confirmed the initial assumptions and even demonstrated the long-term stability of the phase.

“It is a difficult task and only theoretical predictions are possible, but the existence of this new Pu(V) solid phase, which is stable, will have to be taken into account from now on,” Dr. Kvashnina said.

“It will change, for sure, the theoretical predictions of plutonium behavior in the environment over a period of million years.”

The findings were published in the journal Angewandte Chemie.

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Kristina Kvashnina et al. A novel meta-stable pentavalent plutonium solid phase on the pathway from aqueous Pu(VI) to PuO2 nanoparticles. Angewandte Chemie, published online October 17, 2019; doi: 10.1002/anie.201911637

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