​Deep eutectic solvents (DES) are mixtures of molecular and ionic compounds that melt at a temperature lower than their constituent parts, forming a partially ionic liquid phase. They are often based around choline chloride or urea, and are potentially a more environmentally friendly replacement for traditional organic solvents. 

The inclusion of lanthanide ions has the potential to produce a DES that is task specific, and that could be used as a precursor to lanthanide-containing materials such as magnets, lasers and superconductors. This study describes the synthesis of the first lanthanide DES based on a mixture of urea and lanthanide nitrate hydrates, and its structural resolution using X-ray diffraction and neutron scattering. 

The researchers found that, in their cerium (Ce) based DES, the nanostructure is dominated by Ce-O interactions, forming a variety of [-Ce-NO3-] networks. The formation of this ionic pseudophase causes the co-creation of a molecular rich pseudophase alongside it. This insight into the structure of these liquids helps to explain some of their unusual properties, and challenges the definition of what makes a DES.  ​