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CHEMISTS at the University of Nottingham have created a uranium nitride compound that is stable at room temperature, something which has not previously been achieved.

The researchers say that the complex, a uranium (V) atom triple-bonded to a nitrogen atom, could potentially lead to alternatives to mixed oxide uranium nuclear fuels, as uranium nitride compounds have higher densities, melting points, and thermal conductivities, as well as a more sustainable production process. The researchers, led by associate professor of chemistry Stephen Liddle, say that their compound could be stored in crystalline form in jars.

The preparation of uranium nitride has previously required very high temperatures to produce, and was only stable at temperatures below 5K, making it very difficult to handle. However, the process developed by Liddle and his colleagues occurs at fairly mild conditions.

Their technique involves wrapping a uranium atom with a nitrogen ligand, which is an organic molecule weakly bonded to a metal. This effectively creates a ‘pocket’ in which the nitride ion can sit, stabilised by a sodium cation, forming sodium azide (NaN₃). The reactive uranium ‘grabs’ one of the nitrogen atoms, leaving a sodium cation, which is then removed using a type of crown ether. This leaves the stable triple-bonded uranium-nitrogen complex.

The structure of the complex created was confirmed using a variety of techniques including electron paramagnetic resonance (EPR) spectroscopy and ¹⁵N-isotopic labelling.

“A major motivation for doing this work was to help us to understand the nature and extent of the covalency in the chemical bonding of uranium. This is fundamentally interesting and important because it could help in work to extract and separate the 2–3% of the highly radioactive material in nuclear waste,” says Liddle.

Science DOI: 10.1126/science.1223488