Uranium trichloride exhibits transient covalency when hot

At extremely hot temperatures, the uranium–chlorine bonds in uranium trichloride shrink and the compound exhibits transient covalency, new research shows.

To improve the efficiency of nuclear reactors, uranium trichloride is being considered as a liquid alternative to current nuclear fuel. In the solid state, uranium’s 5f orbitals interact with ligand orbitals to form covalent interactions. However, interactions between uranium and ligand orbitals in the molten state, as used in liquid nuclear fuel, are poorly understood. This lack of understanding is due to the extreme conditions necessary to mimic a nuclear reactor and uranium’s radioactivity. ‘The difficulty associated with these measurements can’t really be understated,’ comments Stefan Minasian, an actinide chemist at Lawrence Berkely National Laboratory in the US, who wasn’t involved in the research.

Now, a research team surrounding Alex Ivanov and Santanu Roy from the Oak Ridge Laboratory in the US has conducted neutron scattering and computational experiments on uranium trichloride at 298K and 1173K to compare its structure in the solid and molten states.

They observed that uranium trichloride had transient covalency at 1173K. This phenomenon sees the ligands oscillate at very high speeds and when they are close to the uranium they form metastable covalent bonds.

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Understanding the chemistry of molten uranium trichloride is crucial for developing next-generation liquid nuclear fuels

Source: © Oak Ridge National Laboratory, U.S. Dept. of Energy/Dmitry Maltsev

High-temperature-induced shrinkage of the first coordination sphere around uranium(III) enables better overlap of the chlorine lone pairs with the uranium(III) acceptor orbitals

Source: © Oak Ridge National Laboratory, U.S. Dept. of Energy/Alex Ivanov

Santanu Roy and Alexander Ivanov

Source: © Oak Ridge National Laboratory, U.S. Dept. of Energy/Genevieve Martin

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The uranium–chlorine bonds shrink from 2.92Å in the solid state to 2.781Å upon heating. This decrease results in stronger overlap between uranium’s 5f orbitals and the ligand orbitals, increasing the degree of covalency in these bonds.

At extremely hot temperatures, it would be expected that the uranium–chloride bonds shrink equally, on average. However, the research team observed something unusual; the uranium–chlorine bonds shrunk by varying degrees. There was a larger proportion of short bonds, which explains the overall increased covalency. But longer bonds were also present, exhibiting more ionic character compared to their shorter counterparts. Roy explains that the chlorides are ‘not just structurally, [but] dynamically they’re different too. The short one shows oscillatory behaviour, and the other one is moving around,’ – a quite different situation to standard salts like sodium chloride.

Following this investigation, Ivanov says the team plans to experiment with other uranium chlorides as continued research can ‘predict fundamental properties based on the atomic structure of the salts and inform the future [nuclear] industry and help build [new] reactors’.